Fig. 2

Programmed stiffness of photo-curable hydrogels. a Bright-field optical image of a stiff line feature (UV exposure dosage of 72 mJ cm⁻²) in a soft hydrogel background (UV exposure dosage of 44 mJ cm⁻²). Scale bar is 100 μm. The total thickness is ~500 μm to improve the accuracy of Hertz-model-based AFM stiffness measurement. Matrix stiffness (b) and surface topography (c) mappings via AFM measurement reveal sharp contrast in stiffness (stiff line: E ~ 11 kPa; soft background: E ~ 5 kPa) but small topography variations ( < 600 nm) for a total thickness of ~500 μm. An AFM cantilever with a 10 μm polystyrene sphere attached and a pre-calibrated stiffness of 0.09 N/m was used for the stiffness results reported here. The force distance curves were recorded and the Hertz-model was used to calculate the stiffness of the polymerized matrix. d Quantification of stiffness and geometry variation shows that stiffness (solid black circles) increases from 2–15 kPa with the increase of UV exposure dosage, while the feature height variation (open blue circles) is limited to 2% over UV exposure dosages from 25.5–84 mJ cm⁻². Error bars represent standard deviation (s.d.); n = 5 samples per condition