Precision Stress Engineering in Tensegrity-Inspired Nanoarchitectures Enabled by Size-Affected Shrinkage.

Residual stress networks offer a powerful means to enhance mechanical properties, but controlling them at the nanoscale remains challenging. Here, we introduce a method to create prestressed tensegrity-inspired nanoarchitectures, i.e., nano-tensegrities, by exploiting a previously uncharacterized size-affected shrinkage phenomenon. We discover that the shrinkage of acrylate-based polymers during pyrolysis has a power-law dependence on size. This size-effect arises due to increased residual oxygen-containing groups in larger-dimension specimens. Leveraging this effect, we use two-photon lithography to fabricate polymer structures with thicker "bar" and thinner "tendon" members and pyrolyze them to create prestressed glassy carbon nano-tensegrities. Using combined experiments and numerical modeling, we demonstrate pyrolyzed structures retain their designed state of prestress, which can then be precisely controlled by tuning the bar-to-tendon diameter ratio. Prestress is shown to considerably enhance stiffness - up to a two-and-a-half-fold increase in the structures studied here - but can lead to buckling in excessively stressed slender members. We evaluate the effect of architecture and slenderness on the limits of prestressability and analyze corresponding changes in mechanical performance. This work establishes a method to precisely program 3D residual stress into metamaterials at the nanoscale, enabling a new class of mechanically tunable nanoarchitectures.