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  1. The effect of nanoscale defects on nanoparticle dynamics in defective tetra-poly(ethylene glycol) (tetra-PEG) hydrogels is investigated using single particle tracking. In a swollen nearly homogeneous hydrogel, PEG-functionalized quantum dot (QD) probes with a similar hydrodynamic diameter ( d h = 15.1 nm) to the mesh size (〈 ξ s 〉 = 16.3 nm), are primarily immobile. As defects are introduced to the network by reaction-tuning, both the percentage of mobile QDs and the size of displacements increase as the number and size of the defects increase with hydrolysis time, although a large portion of the QDs remain immobile. To probe the effect of nanoparticle size on dynamics in defective networks, the transport of d h = 47.1 nm fluorescent polystyrene (PS) and d h = 9.6 nm PEG-functionalized QDs is investigated. The PS nanoparticles are immobile in all hydrogels, even in highly defective networks with an open structure. Conversely, the smaller QDs are more sensitive to perturbations in the network structure with an increased percentage of mobile particles and larger diffusion coefficients compared to the larger QDs and PS nanoparticles. The differences in nanoparticle mobility as a function of size suggests that particles of different sizes probe different length scales of the defects, indicating that metrics such as the confinement ratio alone cannot predict bulk dynamics in these systems. This study provides insight into designing hydrogels with controlled transport properties, with particular importance for degradable hydrogels for drug delivery applications. 
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