Oxidation stiffening of PDMS microposts

Abstract

This paper presents guidance for analyzing the stiffness changes of micropost arrays after oxidation. Numerous mechanobiology studies exploit micropost arrays to quantify cell-generated micropost displacements and traction forces. Cell adhesion requires surface functionalization typically facilitated by an oxidation step, which can form a glassy skin. We estimate the material properties of this skin, analytically calculate the stiffness of oxidized microposts and compare it with direct measurements. We find correction factors increase exponentially with decreasing micropost diameter. Ignoring this oxidation stiffening of microposts results in more than 200% underestimation in micropost stiffness as well as the cellular traction forces extracted from micropost deflections.

For whom the cells pull: Hydrogel and micropost devices for measuring traction forces

“…For example, some formulations of polyacrylamide exhibit time-dependent viscoelasticity [71] or poroelastic properties [72, 73] that should be considered when measurements address cell-generated deformations of the substrate exerted at different rates: fast or slow cell contraction. Micropost effective stiffness differently varies with distinct types of post-fabrication treatments of the PDMS surface [74]. In general, the effective post stiffness of a set of representative microposts in an array can be directly calibrated using a force sensor to observe top displacement under an applied transverse load.…”

“…Protein micropatterns on glass can also be developed with methodologies such as microcontact printing by coating a PDMS block with features to stamp proteins onto the glass [112], lift-off of a photoresist layer and glass passivation with PLL-g-PEG [113], and UV micropatterning of glass homogeneously coated with PLL-g-PEG [114]. These micropatterns can be transferred to hydrogel surfaces for TFM measurements of cells confined to specific sizes or shapes [74]. …”

“…Surface oxidation of PDMS is the most common method to induce protein adsorption to PDMS surfaces [93] or to prepare PDMS for microcontact printing (Figure 7). The most common methods to oxidize PDMS surfaces include oxygen plasma treatment [74], UV-ozone deposition [16], incubation in piranha solution [116], and chemical deposition [117]. Oxidation alters micropost stiffness so this must be taken into consideration when calculating cell-generated forces from micropost bending after this treatment [74].…”

“…The most common methods to oxidize PDMS surfaces include oxygen plasma treatment [74], UV-ozone deposition [16], incubation in piranha solution [116], and chemical deposition [117]. Oxidation alters micropost stiffness so this must be taken into consideration when calculating cell-generated forces from micropost bending after this treatment [74]. Adsorption of ECM to oxidized PDMS alone does not induce strong adhesions of cells.…”