A coarse-grained molecular dynamics model for polyrotaxane based 3D printable hydrogels: At equilibrium and under steady shear

The suitability of a hydrogel as an extrusion-based 3D printable ink is primarily governed by its rheological properties. Polyrotaxanes (PR), composed of cyclodextrin (CD) threaded onto polyethylene glycol (PEG) axles, exhibit the shear-thinning and self-healing characteristics desired to form a robust direct-ink-writing 3D printable ink. These attributes stem from the inter-chain crystalline domains formed by CD aggregation, which can be ruptured under stress and reform in quiescent conditions. To better understand the formation and disruption of these domains through the extrusion process, we have used coarse-grained non-equilibrium molecular dynamics simulations to elucidate the crystallization and rheological behavior of PR hydrogels at equilibrium and under steady shear. Our model reproduces the experimentally determined crystal morphology of dilute CD-PEG based PR solutions and hydrogels and enabled us to attain molecular level insight into the effect of shear stress on the crystalline domains. We determined that the shear rate and solution concentration affect the composition and distribution of the crystalline domains during shear and observe the reformation of the network after the cessation of flow. We expect our results to aid in the rational design of new 3D printable hydrogels and extrusion methodology.