Surface Technology with Cold Microplasmas

Abstract

Various new plasma‐based surface technological processes are made feasible by localizing atmospheric‐pressure discharges to predefined volumes with sub‐millimeter linear dimensions. So‐called Plasma Printing processes use cold discharges in microcavities formed temporarily by contacting a substrate with a suitably designed kind of plasma stamp. Aside from dielectric barrier discharges driven by mid‐frequency (MF) AC voltages, cold microplasmas can also be sustained in arrangements without a dielectric barrier, if RF excitation is used. The modification or coating of internal surfaces in already sealed microfluidic systems promises the achievement of a wide range of physico‐chemical surface properties which are difficult to attain by wet‐chemical or low‐pressure plasma processes. Using a proper electrode arrangement, the coating or modification can be localized to a selected segment of a microfluidic system.magnified image

Surface patterning of bonded microfluidic channels

“…This was attributed to flow effects, where plasma species generated an upstream deposit on downstream plasma activated surfaces ͑i.e., at the electrodes͒. Klages et al stated that regions as small as 1 mm can be modified using refined electrode and gas configurations, 30 although an absolute limit was not determined in their study.…”

“…Recently, plasma patterning of microfluidic channels has received significant attention. [28][29][30][31]46,71,72 In most cases, microchannel patterning was achieved prebonding using appropriate masks or directed jets; however, only a few examples of postbonding treatment have been reported. [28][29][30][31]72,73 Postbonding plasma modification of polystyrene microchannels has been demonstrated by Evju et al 28 Platinum electrodes were engineered in the microchannel to generate an electric field along the length of the channel, Fig.…”

“…For greater flexibility and patterning, Klages et al 29,30 demonstrated plasma treatment of bonded channels using an electric field perpendicular to the direction of the microchannel, cf. Fig.…”

“…A range of alternative methods has now been developed to modify internal channel surfaces after bonding, which ensures that the treatment maintains its functionality for the desired application. Most of these methods draw on knowledge from conventional techniques of surface modification, with some degree of adaptation to microscale confinement, e.g., photolithography [25][26][27] or microplasmas, [28][29][30][31] while other methods are emerging which take advantage of specific physical behavior in microchannels, e.g., laminar and capillary flow. 26,27,32,33 This Review discusses prominent examples of postbonding channel patterning that are, or have the potential to become, routine tools for the microfluidics community.…”

Microplasma Array Patterning of Reactive Oxygen and Nitrogen Species onto Polystyrene

“…Current methods for surface patterning, including those utilizing low-pressure plasmas [13][14][15] and chemical vapor deposition [16], usually require the use of physical masks [17,18]. Alternatively, microplasmas can be used for localized surface modification using a method known as "plasma printing" [19][20][21][22][23][24][25]. Here, spatially resolved modification of surface chemistry is achieved through the contact between the substrate and the plasma stamp.…”