Thermally Insulated Sub-100 μm Aerogel Fibers Obtained by Including Chemically Modified and Aligned Thermoplastic Polyurethane Nanofibrils and a Prepolymerized Silica Precursor

Aerogel fibers are promising for advanced thermal management due to their ultralight weight, low thermal conductivity, and flexibility, yet their mechanical fragility and limited scalability hinder practical adoption. Here, we demonstrate a continuous, scalable spunbonding–gel-spinning process to fabricate sub-100 μm silica aerogel fibers reinforced with in situ-fibrillated thermoplastic polyurethane (TPU) and cellulose nanofibers (CNFs). This continuous method combines twin-screw extrusion, shear-induced spinning, and hot-stretching to generate elongational flow and uniaxial alignment of in situ nanofibrillated TPU, as confirmed by SEM. Surface functionalization of TPU and CNF enabled strong interfacial adhesion and segmental packing, induced mechanical anisotropy, enhanced structural integrity, and produced uniaxially aligned nanofibrils embedded within the aerogel matrix. Structural tuning of the silica aerogel phase produced a cross-linked network with 30 nm pores, minimizing transverse thermal conductivity. The resulting aerogel fibers exhibit a thermal conductivity of 27.7 mW/m·K, effusivity of 38.2 W·s0.5/m2·K, and elongation at break of 188%. Evaluating the modulus using the Halpin–Kardos approach resulted in an unprecedented stiffness–density scaling exponent (n = 0.78), indicating exceptional mechanical retention at low densities. Structural and thermal modeling revealed that the hierarchical nanofibril–aerogel architecture suppresses conduction and radiation losses while enhancing load transfer and geometry stability. This scalable approach overcomes key limitations of conventional aerogel fibers, offering a pathway to high-performance insulation for wearable protection, aerospace, construction, and energy-efficient systems.