Viscoelastic characterization of pH-sensitive 2-hydroxyethyl methacrylate (2-dimethylamino) ethyl methacrylate HEMA-DMAEMA hydrogels

Abstract The pH sensitive 2-hydroxyethyl methacrylate (2 - dimethylamino) ethyl methacrylate, (HEMA-DMAEMA) stimuli-responsive hydrogel is used in microfluidic devices as sensors and actuators. This hydrogel responds to an acidic external environment with a subsequent volume change. This actuation requires the hydrogel to be in an acidic environment to remain in its swollen state. This study characterizes this hydrogel in terms of material functions such as the storage and loss modulus, and the loss tangent tan δ in acidic and basic external environments. For the acidic environment (3.0 pH) the storage modulus was 41 ± 9.1 kPa, the loss modulus was 2.7 ± 0.62 kPa and tan δ was 0.069 ± 0.017 at 1 Hz and 1% strain and showed minimal frequency dependence from 0.1 Hz to 100 Hz. In the basic environment (11.0 pH) the storage modulus was 12 ± 1.6 kPa, the loss modulus was 5.0 ± 0.55 kPa and tan δ was 0.42 ± 0.023 at 1 Hz and 1% strain and displayed more pronounced frequency dependence between 0.1 Hz and 100 Hz.