Robocasting of Cu2+ & La3+ doped sol–gel glass scaffolds with greatly enhanced mechanical properties: Compressive strength up to 14 MPa

Abstract

This research details the successful fabrication of scaffolds by robocasting from high silica sol-gel glass doped with Cu 2+ or La 3+ . The parent HSSGG composition within the system SiO 2 -CaO-Na 2 O-P 2 O 5 [67% Si -24% Ca -5% Na -4% P (mol%)] was doped with 5 wt% Cu 2+ or La 3+ (Cu5 and La5). The paper sheds light on the importance of copper and lanthanum in improving the mechanical properties of the 3-D printed scaffolds. 1 h wet milling was sufficient to obtain a bioglass powder ready to be used in the preparation of a 40 vol% solid loading paste suitable for printing. Moreover, Cu addition showed a small reduction in the mean particle size, while La exhibited a greater reduction, compared with the parent glass. Scaffolds with macroporosity between 300 and 500 mm were successfully printed by robocasting, and then sintered at 800°C. A small improvement in the compressive strength (7-18%) over the parent glass accompanied the addition of La. However, a much greater improvement in the compressive strength was observed with Cu addition, up to 221% greater than the parent glass, with compressive strength values of up to $14 MPa. This enhancement in compressive strength, around the upper limit registered for human cancellous bones, supports the potential use of this material in biomedical applications. Statement of Significance3D porous bioactive glass scaffolds with greatly improved compressive strength were fabricated by robocasting from a high silica sol-gel glasses doped with Cu 2+ or La 3+ . In comparison to the parent glass, the mechanical performance of scaffolds was greatly improved by copper-doping (>220%), while a modest increase of $9% was registered for lanthanum-doping. Doping ions (particularly La 3+ ) acted as glass modifiers leading to less extents of silica polymerisation. This favoured the milling of the glass powders and the obtaining of smaller mean particle sizes. Pastes with a high solid loading (40 vol%) and with suitable rheological properties for robocasting were prepared from all glass powders. Scaffolds with dimensions of 3 Â 3 Â 4 mm and macro-pore sizes between 300 and 500 mm were fabricated.

A Comparison of Bioactive Glass Scaffolds Fabricated ‎by Robocasting from Powders Made by Sol–Gel and Melt-Quenching Methods

“…The high porosity of sol-gel-derived bioactive glass powders created even greater difficulties in the paste preparation process, as a significant part of the dispersing liquid is absorbed and trapped in the pores, reducing the flow and stability of the paste/ink. Due to these issues, there have been far fewer reports on the robocasting of bioactive glass scaffolds compared to bioceramics, and this was limited entirely to glass compositions prepared by melt-quenching [48,49], until the successful robocasting of sol-gel-derived bioactive glass scaffolds was reported for the first time in 2019 by Ben-Arfa et al [23,24].…”

“…On the other hand, it was demonstrated by Ben Arfa et al [31,55] that a simple, cheap rapid milling machine was sufficient to mill glass granules for robocasting in 2 h, and that highly expensive and sophisticated milling machines (attrition mills, planetary mills) are not necessary. These authors also used ethanol as the milling medium and, employing a BPR of 10 and LPR of 1.5, reduced the milling time further to just 1 h to prepare a sol-gel glass robocasting paste [23,24].…”

“…A high solids loading significantly increases the viscosity of the paste, as well as the mechanical properties of the green body and the final sintered scaffold [60]. The glass synthesis method used does not present a great change in the maximum solids loading, with a narrow variation in range of between 35 and 45 vol% for 45S5 [15,48] and HSSGG robocast glasses [23,24], reflecting the need for a reasonably high solids loading value for a successful robocasting glass paste/ink.…”

“…Coagulants are also types of polymers that act as physical glue, providing resistance to the external forces exerted on the extruded struts due to the load of the subsequent layer above. Such mixtures were used in the early robocasting of bioactive glasses, but these days, a single additive is mostly used, such as carboxymethyl cellulose (CMC) [23,24,48] or Pluronic F-127 [8,12], as a combined dispersant, plasticizer, and coagulant. The use of a single additive enables a fast and easy process, especially in the case of CMC, which can be used at room temperature, whereas Pluronic F-127 needs a non-ambient temperature to prepare the suspension.…”

“…This is because sol-gel-derived bioactive glasses have proven problematic for robocasting, as the high specific surface area and porous structure of the glass powder results in absorption of a significant portion of the dispersing liquid from the suspension, impeding paste/ink formulation and greatly reducing printability. The first robocast sol-gel-synthesized bioactive glass scaffolds were only reported in 2019 [23,24]. These were of a quaternary high-silica sol-gel glass (HSSGG) with a low sodium composition of 64.4 wt% SiO 2 , 4.9 wt% Na 2 O, 21.5 wt% CaO, and 9.1 wt% P 2 O 5 [24].…”