Strong, Machinable, and Insulating Chitosan–Urea Aerogels: Toward Ambient Pressure Drying of Biopolymer Aerogel Monoliths

Abstract

Biopolymer aerogels are an emerging class of materials with potential applications in drug delivery, thermal insulation, separation, and filtration. Chitosan is of particular interest as a sustainable, biocompatible, and abundant raw material. Here, we present urea-modified chitosan aerogels with a high surface area and excellent thermal and mechanical properties. The irreversible gelation of an acidic chitosan solution is triggered by the thermal decomposition of urea at 80°C through an increase in pH and, more importantly, the formation of abundant ureido terminal groups. The hydrogels are dried using either supercritical CO 2 drying (SCD) or ambient pressure drying (APD) methods to elucidate the influence of the drying process on the final aerogel properties. The hydrogels are exchanged into ethanol prior to SCD, and into ethanol and then heptane prior to APD. The surface chemistry and microstructure are monitored by solid-state NMR and Fourier transform infrared spectroscopy, scanning electron microscopy, and nitrogen sorption. Surprisingly, large monolithic aerogel plates (70 × 70 mm 2 ) can be produced by APD, albeit at a somewhat higher density (0.17−0.42 g/cm 3 ). The as prepared aerogels have thermal conductivities of ∼24 and ∼31 mW/(m•K) and surface areas of 160−170 and 85−230 m 2 /g, for SCD and APD, respectively. For a primarily biopolymer-based material, these aerogels are exceptionally stable at elevated temperature (TGA) and char and self-extinguish after direct flame exposure. The urea-modified chitosan aerogels display superior mechanical properties compared to traditional silica aerogels, with no brittle rupture up to at least 80% strain, and depending on the chitosan concentration, relatively high E-moduli (1.0−11.6 MPa), and stress at 80% strain values (σ 80 of 3.5−17.9 MPa). Remarkably, the aerogel monoliths can be shaped and machined with standard tools, for example, drilling and sawing. This first demonstration to produce monolithic and machinable, mesoporous aerogels from bio-sourced, renewable, and nontoxic precursors, combined with the potential for reduced production cost by means of simple APD, opens up new opportunities for biopolymer aerogel applications and marks an important step toward commercialization of biopolymer aerogels.

Ureido Functionalization through Amine-Urea Transamidation under Mild Reaction Conditions

“…The main advantages are (i) the non-toxicity, biocompatibility, low cost and availability of urea as a feedstock; (ii) the high conversion with a wide range of amine substrates; (iii) the compatibility with water and ethanol as a solvent system; and (iv) the relatively mild reaction conditions (<100 °C, atmospheric pressure, no catalyst, wide range of pH values). These advantages are particularly relevant for chitosan aerogel production: the biopolymer aerogel field experiences a strong trend toward eliminating toxic cross-linkers, e.g., aldehydes in the case of chitosan aerogels [ 47 , 66 , 82 ]. The main downsides are (i) relatively low reaction rates (ca.…”

“…For non-isocyanate polyurethane synthesis, amine compounds/cyclic carbonates [ 33 , 34 , 35 , 36 ] and bio-based molecules [ 37 , 38 , 39 , 40 ] such as fatty acids [ 41 ] and vegetable oils [ 42 , 43 , 44 , 45 , 46 ] are of particular interest. For ureido and ureylene formation, urea is a potential green alternative to isocyanates [ 21 , 47 ]. Urea is attractive because it is a non-toxic, abundant reagent, and the synthesis can be carried out in aqueous solutions.…”

“…Recently, ureido functionalization of chitosan, in a low-temperature aqueous process, has enabled the synthesis of high-quality chitosan aerogels [ 47 , 66 ]. The ureido-functionalized chitosan aerogels could even be produced by ambient pressure drying, with outstanding mechanical properties, an exceptional feat in the field of biopolymer aerogels [ 47 ].…”

Insights into the Role of Biopolymer Aerogel Scaffolds in Tissue Engineering and Regenerative Medicine

“…Aerogel is a low-density nanoporous solid material with an ultrafine open pore structure, resulting in extremely large surface areas and low densities [ 6 ]. Aerogels have been prepared from various materials, including biopolymers [ 7 , 8 ], resulting in a desirable combination of properties suitable for many non-medical applications such as water purification [ 9 , 10 , 11 , 12 ], thermal insulation [ 13 ], acoustic properties [ 14 ], etc., and medical applications such as drug delivery [ 15 , 16 ], biosensing [ 17 , 18 ], tissue scaffolding, and regenerative medicine [ 19 , 20 ]. Various technological approaches have been developed to fabricate aerogel scaffolds, including freeze-drying, phase separation, particulate-leaching, and gas foaming, showing inexpensive approaches and optimized physicochemical property structures [ 21 ].…”

Gradual hydrophobization of silica aerogel for controlled drug release

“…2,3 The aerogels' highly versatile synthesis procedures allow tailoring of the material's properties, such as the bulk density, 4 pore size, 5 and surface chemistry. 6 This exibility makes aerogels a sought-aer material for a great variety of applications, including thermal insulators, 7 absorbents, 8 oilwater separators, 9 llers, 10 electrode materials, 11 catalysts, 12 tissue-scaffolds, 13 stents, 14 enzyme supports, 15 Cherenkov radiators, 16 optical devices 17 and cosmic dust collectors. 18 Among the many tunable characters, one critical feature is the hydrophobicity of the aerogel surface.…”