Abstract
Rapidly growing 3D printing of hydrogels requires network materials which combine enhanced mechanical properties and printability. One of the most promising approaches to strengthen the hydrogels consists of the incorporation of inorganic fillers. In this paper, the rheological properties important for 3D printability were studied for nanocomposite hydrogels based on a rigid network of percolating halloysite nanotubes embedded in a soft alginate network cross-linked by calcium ions. Particular attention was paid to the effect of polymer cross-linking on these properties. It was revealed that the system possessed a pronounced shear-thinning behavior accompanied by a viscosity drop of 4–5 orders of magnitude. The polymer cross-links enhanced the shear-thinning properties and accelerated the viscosity recovery at rest so that the system could regain 96% of viscosity in only 18 s. Increasing the cross-linking of the soft network also enhanced the storage modulus of the nanocomposite system by up to 2 kPa. Through SAXS data, it was shown that at cross-linking, the junction zones consisting of fragments of two laterally aligned polymer chains were formed, which should have provided additional strength to the hydrogel. At the same time, the cross-linking of the soft network only slightly affected the yield stress, which seemed to be mainly determined by the rigid percolation network of nanotubes and reached 327 Pa. These properties make the alginate/halloysite hydrogels very promising for 3D printing, in particular, for biomedical purposes taking into account the natural origin, low toxicity, and good biocompatibility of both components.
Highlights
Three-dimensional (3D) printing represents an additive technology of fabricating 3D objects by successive layer-by-layer addition of material following the digital design [1]
By Transmission Electron Microscopy (TEM), it was shown that the halloysite nanotubes (HNTs) under study had an average length L of 1000 nm and an average outer diameter D of ca. 50 nm (Figure 1), which resulted in a high aspect ratio L/D of 20
Rheological studies were of performed in order cluding two laterally arranged chain fragments in the alginate hydrogels
Summary
Three-dimensional (3D) printing represents an additive technology of fabricating 3D objects by successive layer-by-layer addition of material following the digital design [1]. Having been extracted from the cell walls of brown algae, it is non-toxic, biocompatible, and biodegradable. It can form hydrogels employed for 3D printing [2]. The alginate hydrogels are produced using multivalent cations (e.g., Ca2+ ) as cross-linkers [5] In this case, the cross-linking was instantaneous and proceeded at very mild conditions, permitting biological objects such as living cells, enzymes, or DNA to be incorporated into the gels without damage [6]. In the cross-linking by Ca2+ , mainly polyguluronate GG blocks of the polymer chains are involved yielding junction zones with an “egg-box” structure [7] and leaving alginate residues in other blocks without cross-linking Such large junction zones provide rather strong cross-links while still keeping reversible character due to their non-covalent nature
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