HIGHLIGHTS • The study reveals the great potential of metal-organic framework (MOF)-based nanocomposites in thermal insulation and fire retardancy applications. • A nanoengineering approach was developed to process MOFs into freestanding, mechanically strong, and elastic aerogels, which may boost the fundamental research and practical applications of MOFs in these areas. ABSTRACT Metal-organic frameworks (MOFs) with high microporosity and relatively high thermal stability are potential thermal insulation and flame-retardant materials. However, the difficulties in processing and shaping MOFs have largely hampered their applications in these areas. This study outlines the fabrication of hybrid CNF@MOF aerogels by a stepwise assembly approach involving the coating and cross-linking of cellulose nanofibers (CNFs) with continuous nanolayers of MOFs. The cross-linking gives the aerogels high mechanical strength but superelasticity (80% maximum recoverable strain, high specific compression modulus of ~ 200 MPa cm 3 g −1 , and specific stress of ~ 100 MPa cm 3 g −1).
Currently, there are limited approaches to tailor 3D scaffolds cross-linked with a stable covalent C−C bond that does not require any catalysts or initiators. We present here the first hydrogels employing aldol condensation chemistry that exhibit exceptional physicochemical properties. We investigated the aldol-cross-linking chemistry using two types of aldehyde-modified hyaluronic acid (HA) derivatives, namely, an enolizable HA-aldehyde (HA-Eal) and a nonenolizable HA-aldehyde (HA-Nal). Hydrogels formed using HA-Eal demonstrate inferior crosslinking efficiency (due to intramolecular loop formation), when compared with hydrogels formed by mixing HA-Eal and HA-NaI leading to a cross-aldol product. The change in mechanical properties as a result of cross-linking at different pH values is determined using rheological measurements and is interpreted in terms of molecular weight between cross-links (M c ). The novel HA cross-aldol hydrogel demonstrate excellent hydrolytic stability and favorable mechanical properties but allow hyaluronidasemediated enzymatic degradation. Interestingly, residual aldehyde functionality within the aldol product rendered the tissueadhesive properties by bonding two bone tissues. The aldehyde functionality also facilitated facile post-synthetic modifications with nucleophilic reagents. Finally, we demonstrate that the novel hydrogel is biocompatible with encapsulated stem cells that show a linear rate of expansion in our 3−6 days of study.
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