Polymers have assumed the leading role as substrate materials for microfluidic devices in recent years. They offer a broad range of material parameters as well as material and surface chemical properties which enable microscopic design features that cannot be realised by any other class of materials. A similar range of fabrication technologies exist to generate microfluidic devices from these materials. This review will introduce the currently relevant microfabrication technologies such as replication methods like hot embossing, injection molding, microthermoforming and casting as well as photodefining methods like lithography and laser ablation for microfluidic systems and discuss academic and industrial considerations for their use. A section on back-end processing completes the overview.

Polymer microfabrication technologies for microfluidic systems

Natural superhydrophobic surfaces are often thought to have antibiofouling potential due to their self-cleaning properties. However, when incubated on cicada wings, Pseudomonas aeruginosa cells are not repelled; instead they are penetrated by the nanopillar arrays present on the wing surface, resulting in bacterial cell death. Cicada wings are effective antibacterial, as opposed to antibiofouling, surfaces.

Natural Bactericidal Surfaces: Mechanical Rupture of Pseudomonas aeruginosa Cells by Cicada Wings

Lab-on-a-chip devices

Plasma processing is used to fabricate super hydrophilic or super hydrophobic polymeric surfaces by means of O-2 plasma etching of two organic polymers, namely, poly(methyl methacrylate) (PMMA) and poly(ether ether ketone) (PEEK); a C4F8 plasma deposition follows O-2 plasma etching, if surface hydrophobization is desired. We demonstrate high aspect ratio pillars with height ranging from 16 nm to several micrometers depending oil the processing time, and contact angle (CA) close to 0 degrees after O-2-Plasma treatment or CA of 153 degrees (with CA hysteresis lower than 5 degrees) after fluorocarbon deposition. Super hydrophobic surfaces are robust and stable in time; in addition, aging of super hydrophilic surfaces is significantly retarded because of the beneficial effect of the nanotextured topography. The mechanisms responsible for the plasma-induced PMMA and PEEK surface nanotexturing are unveiled through intelligent experiments involving intentional modification of the reactor wall material and X-ray photoelectron spectroscopy, which is also used to study the surface chemical modification in the plasma. We prove that control of plasma nanotexture call be achieved by carefully choosing the reactor wall material.

Mechanisms of oxygen plasma nanotexturing of organic polymer surfaces: from stable super hydrophilic to super hydrophobic surfaces.

https://arrow.tudublin.ie/cgi/viewcontent.cgi?article=1120&context=schfsehart

In-Package Atmospheric Pressure Cold Plasma Treatment of Strawberries

A rapid, easy-to-implement, and potentially large-scale production method for fabricating high-aspect-ratio columnar-like nanostructures on poly(dimethylsiloxane) (PDMS) is demonstrated. Plasma treatment of PDMS under appropriate conditions in SF6 gas, followed by plasma-induced fluorocarbon film deposition, results in PDMS surfaces of fully controlled wetting properties, geometrical characteristics leading to robust superhydrophobic surfaces, and transparency. Potential applications to microfluidic devices are outlined.

https://iopscience.iop.org/article/10.1088/0957-4484/17/15/062/pdf

Nanotexturing of poly(dimethylsiloxane) in plasmas for creating robust super-hydrophobic surfaces

A low cost, low temperature process for sealing microfluidic devices composed of at least one organic polymeric substrate is presented. The process is based on the surface modification of the organic substrate by means of a silane solution, resulting in irreversible bonding. It is a generic method of bonding polymeric/plastic substrates, bare or structured ones, such as poly(methylmethacrylate) (PMMA), polystyrene (PS) or epoxy-type polymers, to Si-containing substrates, such as poly(dimethylsiloxane) (PDMS), Si and glass. In the case that bonding between organic polymer (PMMA, PS, etc) substrates is desired, an intermediate thin PDMS layer is required.

https://iopscience.iop.org/article/10.1088/0960-1317/19/1/015007/pdf

A low temperature surface modification assisted method for bonding plastic substrates

Effect of surface nanostructuring of PDMS on wetting properties, hydrophobic recovery and protein adsorption

Plasma Etching of Poly(dimethylsiloxane): Roughness Formation, Mechanism, Control, and Application in the Fabrication of Microfluidic Structures

We review our recent work on the deterministic (top down) micro/nano structuring and stochastic (random) micro/nano texturing of polymeric materials for a variety of applications. We create polymeric microfludics/microsystems using plasma processing as well as 'smart' polymer surfaces with controlled wetting and optical properties. First, fabrication of polymeric micro-nanosystems and microfluidics using standard photolithography followed by plasma etching is proposed as an alternative method for polymer microfabrication. Second, plasma induced polymer nanoroughening (nanotexturing) is proposed as an extremely attractive methodology to control the wetting properties of polymers and create superhydrophilic or hydrophobic or robust superhydrophobic polymer surfaces. These surfaces may in addition possess controlled optical properties, such as antireflectance combined with good transmittance. Third, ordered spontaneously formed structures are created on plasma processed polymers. Potential applications are discussed.

M. E. Vlachopoulou

Papers

Nanotexturing of poly(dimethylsiloxane) in plasmas for creating robust super-hydrophobic surfaces

A low temperature surface modification assisted method for bonding plastic substrates

Effect of surface nanostructuring of PDMS on wetting properties, hydrophobic recovery and protein adsorption

Plasma Etching of Poly(dimethylsiloxane): Roughness Formation, Mechanism, Control, and Application in the Fabrication of Microfluidic Structures

Micro and nano structuring and texturing of polymers using plasma processes: potential manufacturing applications