Surface protein gradients generated in sealed microchannels using spatially varying helium microplasma

Abstract

Spatially varied surface treatment of a fluorescently labeled Bovine Serum Albumin (BSA) protein, on the walls of a closed (sealed) microchannel is achieved via a well-defined gradient in plasma intensity. The microchips comprised a microchannel positioned in-between two microelectrodes (embedded in the chip) with a variable electrode separation along the length of the channel. The channel and electrodes were 50 μm and 100 μm wide, respectively, 50 μm deep, and adjacent to the channel for a length of 18 mm. The electrode separation distance was varied linearly from 50 μm at one end of the channel to a maximum distance of 150, 300, 500, or 1000 μm to generate a gradient in helium plasma intensity. Plasma ignition was achieved at a helium flow rate of 2.5 ml/min, 8.5 kVpk-pk, and 10 kHz. It is shown that the plasma intensity decreases with increasing electrode separation and is directly related to the residual amount of BSA left after the treatment. The plasma intensity and surface protein gradient, for the different electrode gradients studied, collapse onto master curves when plotted against electrode separation. This precise spatial control is expected to enable the surface protein gradient to be tuned for a range of applications, including high-throughput screening and cell-biomolecule-biomaterial interactions.

Electrical and optical properties of a gradient microplasma for microfluidic chips

“…This expansion of the discharge glow along the microchannel is consistent with the gradual purging of air with He in the microchannel. It can be concluded that the gradual decrease of light emission along the microchannel, observed in our previous work, is from a He discharge. The rapid decrease of the light emission along the microchannel in this work is from a discharge with an air–He mixture.…”

“…Comparison of the plasma intensity profile along the microchannel in our previous work (Figure a, d = 150 μm) with this study (Figure ) shows a significant difference. In our previous work, the total light intensity (normalised to the intensity at the narrow end of the microchannel) decreases steadily along the microchannel reaching a value of around 0.15 (relative intensity).…”

“…In our previous paper, we demonstrated that microfluidic chips, fabricated with microchannels adjacent to pairs of graded electrodes, can be used to generate microplasmas with gradient emission intensities, which is dependent on the slope of the electrodes . Microplasmas with graded emission intensities were used to produce micro‐scale surface protein gradients in bonded microchannels.…”

“…Consequently, the rate of protein modification is expected to be significantly lower in the tenuous discharge region. With increasing electrode gradient, and weaker electric fields, the filament density is expected to decrease further leading to less protein modification along the microchannel …”

“…However, the characterisation of the microplasma inside the microchannel is challenging; arising from the small scale of the microchannel (50 μm), confined environment and the non‐planar geometry. Previously, we demonstrated the efficacy of using gradient microplasmas to modify and generate protein gradients inside microfluidic chips . In that previous study, we adsorbed a homogeneous coating of bovine serum albumin (BSA) to the microchannel walls.…”