Evaluation of Adhesion Forces for the Manipulation of Micro-Objects in Submerged Environment through Deposition of pH Responsive Polyelectrolyte Layers

Abstract

Optimization of surface treatment for reversible adhesion of micro-objects in liquid environment for the need in microassembly processes is presented. A spherical borosilicate probe and planar oxidized silicon wafer substrates were modified by deposition of pH sensitive polyelectrolyte films through layer-by-layer technique. Branched polyethylenimine (b-PEI) and poly(sodium styrenesulfonate) (PSS) were deposited in alternating manner on surfaces, and the influence of polyelectrolyte concentration, pH of deposition, and number of layers on the adhesion were successively examined. The multilayer buildup was followed by optical reflectometry (OR) and dissipative quartz crystal microbalance (QCM-D). The adhesion forces were monitored in aqueous environment at variable pH values by colloidal probe AFM microscopy. The thermodynamic work of adhesion was derived from the pull-off forces by using the Johnson-Kendall-Roberts (JKR) model and compared to the work of adhesion determined from contact angle measurements. It was found out that they correlate well, however, the values accessed from JKR model were underestimated, which was attributed mainly to the effect of surface roughness. Obtained results have demonstrated that it is possible to achieve repeatable reversible adhesion with the change of pH of submerged environment by appropriately tailoring the surface properties and therefore the prevailing surface forces.

Effect of PEG–PDMAEMA Block Copolymer Architecture on Polyelectrolyte Complex Formation with Heparin

“…Such signal reduction was most significant for PEG 114 PDMAEMA 100 . Similar behavior has been observed also for other multilayers composed of weak and strong polyelectrolyte. − This behavior can be explained by the removal of polyelectrolyte complexes, desorption of ions, rearrangement of the layer or combination of these, and it can depend on polyelectrolyte molar mass, pH − and electrolyte concentration . Additionally, it must be considered that the heparin surface is not solid, but it most likely changes conformation and exchanges water and ions upon polymer addition.…”

“…41−45 This behavior can be explained by the removal of polyelectrolyte complexes, 41 desorption of ions, 42 rearrangement of the layer 43 or combination of these, and it can depend on polyelectrolyte molar mass, 42 pH 41−45 and electrolyte concentration. 44 Additionally, it must be considered that the heparin surface is not solid, but it most likely changes conformation and exchanges water and ions upon polymer addition.…”

pH-Responsive PEG/PAA Multilayer Assemblies for Reversible Adhesion of Micro-Objects

“…Similar forces were measured between a b-PEI (branched polyethylenimine)-treated BSG probe and electrostatic-based multilayer made of b-PEI and polystyrene sulfonate (PSS), namely, (b-PEI/PSS)5PEI multilayer films constructed at different pH. 18 Consequently, hydrogen bonding interaction is of the same magnitude compared to electrostatic interaction and so should be strong enough to grasp a micro-object.…”

“…The standard deviation of pull-in and pull-off forces was calculated from five measurements done at different substrate locations and was less than 10%. The experimental procedure to measure force–distance curves between an unmodified colloidal probe and treated planar substrates in the liquid is described in detail in a previous work . To determine the pull-off forces, i.e., adhesion forces, the local minimum of the AFM retraction profile was identified since it corresponds to the critical force necessary to separate the colloidal probe from the surface.…”

“…The experimental procedure to measure force−distance curves between an unmodified colloidal probe and treated planar substrates in the liquid is described in detail in a previous work. 18 To determine the pull-off forces, i.e., adhesion forces, the local minimum of the AFM retraction profile was identified since it corresponds to the critical force necessary to separate the colloidal probe from the surface. Then, the thermodynamic work of adhesion was calculated from the adhesion force measurements using the Johnson, Kendall, and Roberts (JKR) relationship.…”