Characterization of the stiffness gradient hydrogels.

<p>(A) Schematic of AFM hydrogel mechanical measurement: (Cartoon) Illustration of an AFM probing the hydrogel elastic modulus; (Graph) Force–indentation (<i>F</i>–<i>δ</i>) curve obtained as the cantilever tip approached and indented a hydrogel (solid line) at 10 µm s⁻¹.This curve was used to determine the contact point between the tip and the sample (arrow). The rising portion of the curve yielded the Young's modulus (<i>E</i>) of the hydrogel. (B) Hydrogel stiffness variation (circles) produced by a linear irradiation profile (solid line). At the beginning of the fabrication process, the opaque mask is placed at gel position <i>x</i> = 0 mm (indicated by a dashed line). For <i>x</i><0, the hydrogel irradiation (solid line) is constant during the fabrication process. As the fabrication begins, the mask is moved at a constant speed of 15 µm s⁻¹. This results in a decreasing irradiation from the dashed line to the end of the gel. Although the irradiation has a linear profile, the Young's modulus of this hydrogel region decreases non-linearly from 240 kPa to ∼1 kPa. (C) Hydrogel stiffness as a function of irradiation dose. We use this calibration curve to compute the mask speed profile needed to linearize the gel stiffness profile (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046107#pone.0046107.s001" target="_blank">Text S1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046107#pone.0046107.s003" target="_blank">Fig. S2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0046107#pone.0046107.s004" target="_blank">Fig. S3</a>). Error bars represent SE of 4 replicates.