Self-Supporting Nanoclay as Internal Scaffold Material for Direct Printing of Soft Hydrogel Composite Structures in Air

Abstract

Three dimensional (3D) bioprinting technology enables the freeform fabrication of complex constructs from various hydrogels and is receiving increasing attention in tissue engineering. The objective of this study is to develop a novel self-supporting direct hydrogel printing approach to extrude complex 3D hydrogel composite structures in air without the help of a support bath. Laponite, a member of the smectite mineral family, is investigated to serve as an internal scaffold material for the direct printing of hydrogel composite structures in air. In the proposed printing approach, due to its yield-stress property, Laponite nanoclay can be easily extruded through a nozzle as a liquid and self-supported after extrusion as a solid. Its unique crystal structure with positive and negative charges enables it to be mixed with many chemically and physically cross-linked hydrogels, which makes it an ideal internal scaffold material for the fabrication of various hydrogel structures. By mixing Laponite nanoclay with various hydrogel precursors, the hydrogel composites retain their self-supporting capacity and can be printed into 3D structures directly in air and retain their shapes before cross-linking. Then, the whole structures are solidified in situ by applying suitable cross-linking stimuli. The addition of Laponite nanoclay can effectively improve the mechanical and biological properties of hydrogel composites. Specifically, the addition of Laponite nanoclay results in a significant increase in the Young's modulus of each hydrogel-Laponite composite: 1.9-fold increase for the poly(ethylene glycol) diacrylate (PEGDA)-Laponite composite, 7.4-fold increase for the alginate-Laponite composite, and 3.3-fold increase for the gelatin-Laponite composite.

Study of extrudability and standoff distance effect during nanoclay-enabled direct printing

“…If the applied stress is higher than its shear yield stress, it behaves like liquid; otherwise, it behaves like solid. At the nozzle exit, the shear stress drops close to zero, resulting in a rapid liquid to solid-like transition behavior [12,25]. As a result, the extruded Laponite filament has a well-defined morphology with negligible die swelling as shown in Fig.…”

“…As a result, the NIPAAm solution has poor extrudability and cannot be used alone for extrusion-based printing applications. Figure 3b-1 and b-2 shows the extrusion process of the Laponite nanoclay suspension, a Bingham plastic material with the unique yield-stress property [12,13,25,26] a nozzle, the Laponite nanoclay suspension in the nozzle is sheared and behaves as a shear thinning fluid material. If the applied stress is higher than its shear yield stress, it behaves like liquid; otherwise, it behaves like solid.…”

“…However, this fabrication methodology follows the traditional "solidification-while-printing" procedure and has some constraints including the requirement of rapid solidification of liquid build materials in order to retain the printed shape and nozzle clogging. To overcome these constraints, a novel extrusion fabrication approach, nanoclay-enabled self-supporting "printing-thensolidification" [12], has been successfully demonstrated: Laponite nanoclay is used as an internal scaffold material to hold liquid 3D structures in air before solidification; after the entire 3D structure is finished, stimuli are used to cure and solidify it.…”

“…Kang et al [20] printed various alginate, poly(ethylene glycol) diacrylate (PEGDA), and gelatin bioinks under different printing conditions and found that printing conditions including the nozzle diameter, dispensing pressure, path height (standoff distance), path spacing (feed), and printing speed can affect the printing accuracy and resolution of 3D structures. For nanoclay-enabled direct extrusion printing, while the effects of operating conditions on the filament diameter and morphology have been investigated during the extrusion of nanoclay-based hydrogel composite filaments [12], the influence of nanoclay on the extrudability and print quality is still to be further elucidated.…”

“…To investigate the influence of nanoclay additive during nanoclay-enabled direct extrusion printing, Laponite XLG nanoclay is used as both ink material and internal scaffold material. Both Laponite and its nanocomposites have been utilized in different biomedical applications [12,23,24]. As the internal scaffold material during extrusion, Laponite XLG is added to prepare the NIPAAm-Laponite nanocomposite hydrogel precursor to investigate the improvement of the bioink extrudability.…”