Thermal insulation is a key technology for energy conservation in the 21st century. One of the major issues of the current thermal insulation technology is heat flow through transparent parts such as windows and glass walls. Silica aerogels, which consist of a three-dimensional network of 10− 20 nm silica nanoparticles with a porosity of 90− 98%, have attracted much attention as a transparent insulator. 1 Although silica aerogels have high transparency in the visible region and extremely low thermal conductivity, their commercial uses are limited by their brittleness and fragile nature. A variety of approaches to reinforcing the mechanical properties of silica aerogels have been reported, eg, coating polymers at the silica surface, 2, 3 incorporating organic functional groups, 4, 5 penetrating fibrous polymers into silica aerogel matrices, 6, 7 and embedding silica aerogel particles in polymer matrices. 8 However, increasing organic contents inevitably increases structural inhomogeneity at the nanoscale, which results in low transparency and high thermal conductivity. A new category of thermal insulating materials, which has a combination of transparency, flexibility, and a low thermal conductivity, has long been required in this field. In contrast to traditional aerogels consisting of nanoparticle skeletons, some researchers have focused on nanofibrous aerogels composed of entangled nanofibers of soft polymers. 9− 11 These aerogels are expected to have flexibility and become suitable candidates for flexible thermal insulators. Most of the nanofibrous aerogels reported so far were opaque because they had structural inhomogeneity at the scale of>∼ 100 nm. Thus, the synthesis of transparent nanofibrous aerogels has been a major challenge in this field. A recent breakthrough was made by Isogai’s group, who reported a translucent aerogel of oriented cellulose nanofibers. 12 This aerogel had a homogeneous porous structure at the nanoscale and hence a low thermal conductivity. However, the use of oxidative catalysts, eg, 2, 2, 6, 6-tetramethylpiperidine-1-oxyl (TEMPO), in the production of cellulose nanofibers becomes an industrial problem because of its high cost and hazardous properties. An alternative approach is therefore needed to synthesize transparent nanofibrous aerogels without using any high-cost catalysts or oxidants.

Chitosan is a biomass-derived polysaccaride abundant in many raw materials, with a low cost, yet is environmentally friendly, and has high biocompatibility. Several gelation approaches to producing chitosan aerogels, such as physical gelation in basic solutions, 13, 14 cross-linking with hemicellulose citrate, 15 and cross-linking with aldehydes, 16− 19 have been reported. Despite the range of available synthetic methods, these previous reports have dealt only with opaque aerogels with high densities. The chitosan aerogels have been proposed