A nonlinear viscoelastic 3D bellows microspring fabricated with two-photon polymerization

In the past decade, two-photon polymerization (TPP) based 3D printing has gained popularity for prototype fabrication of micro and nanoscale structures and mechanical metamaterials. However, practical considerations including response uniformity, nonlinear behaviors, and material aging effects remain challenges for device maturation and are seldom reported. In this work, a pair of cylindrical microscale bellows springs (~ 100 μm diameter), fabricated with a commercial TPP 3D printer, are shown to exhibit nonlinear stiffness profiles under compression (up to ~ 7%), with distinct linear and nonlinear regions, that change significantly after ~ 9-10 months of open air exposure in ambient lab conditions. Under constant loads (2-10 mN) the springs exhibit a consistent drift ("creep") response (-143 nN/nm) with displacement rates increasing with load and decreasing with dwell time. A viscoelastic response is evident under cyclic subresonant (0.5-1.5 Hz) low amplitude (0.5-1.5 mN) loading with the spring loss angle increasing with frequency. While some of these characteristics can be attributed to the spring design, the results offer compelling motivation for deeper consideration of nonlinear material properties in two-photon polymer resist development.