1. Researchers from Korea investigated the effects of microstructural evolution on the density of carbon nanotube (CNT) fibers, which is crucial for their mechanical properties and applications. 2. The study highlighted the impact of residual chlorosulfonic acid (CSA) on the density and mechanical properties of CNT fibers. 3. The findings suggest that understanding structural evolution is essential for optimizing the performance of CNT fibers in high-strength applications.