Device Fabrication White Paper S2 Jan 2019

Creation of hydrophobic, hydrophilic and functional surfaces

a)

A key requirement of microfluidics fabrication for biomedical devices is the ability to control surface properties after the creation of channels. In this paper we will overview processing solutions for hydrophobic and hydrophilic surface creation which is critical for both active functions of the device as well as for post processing challenges such as bonding, sealing and de-scum. Surface roughness is very important for microfluidic flow characteristics like pressure gradients, friction, heat transfer, turbulence etc., which can be achieved by carefully optimizing the channel fabrication processes as described above. However, an equally important surface technique is the control over its hydrophobicity. Tuning this property over the device surface enables control over fluid flowand interaction. In addition, there also a need to match the hydrophobicity of all the surfaces that fluid comes in contact with to enable uniform interactions and flow. For example: a Si-wafer with SiO x on its surface can be too hydrophilic while a PMMA device too hydrophobic. Such a control not only enables improvement of efficiency in pumping fluids but also important for fabricating structures for operations such as mixing, positioning or separating purely by engineering hydrophilic-hydrophobic surface transitions. This provides the additional advantage of decreasing the size and complexity of devices by avoiding the need for active control measures which usually require additional energy sources or pumps to control the fluid.

b)

Figure 2. Conversion of hydro- phobic parylene PPX-N to hydro- philic using an Oxygen Plasma treatment on the Plasmalab 80 plus (reproduced from 1 CC BY 4.0). (b) Zeonor, a common cyclic olefin copolymer (COC) is ren- dered hydrophilic by deposition of tetraethylorthosilicate-acrylic acid films in a Plasmalab 100 PECVD 2 . (Reproduced with permissions from J. Chem. Educ. 94, 221, (2017). Copyright 2018 American Chemical Society).

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Functional layers for sensing

Nitride/oxide passivation

Metal

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Hydrophobic coatings

Figure 1 . Schematic illustration of the function of organic coatings

A hydrophobic surface does not allow water to flow into it without additional pressure while a channel with a hydrophilic surface naturally allows water penetration. This effect can be used to control the flowof liquid. Hydrophobic surfaces are not only useful for microfluidic device operation but also in various other facets of biomedical device fabrication such as mould fabrication, bonding, moisture barriers etc. There are several ways to engineer hydrophilic and hydrophobic surfaces using Oxford Instruments Plasma Technology processes as follows: 1. Surfaces functionalized with polar molecules are generally hydrophilic due to their ability to bind water molecules through hydrogen bonding. Such surfaces attract water through capillary action. This can be achieved by treating most surfaces to Oxygen plasma which creates –OH groups on the surface to aid hydrogen bonding. (Figure 2 a, b). Hydrophilic surfaces can also be created by deposition of films with tailored surface functional groups to strongly promote hydrogen bonding.

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