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. 2024 May 15;20(19):4021-4034.
doi: 10.1039/d4sm00135d.

Flax fibre reinforced alginate poloxamer hydrogel: assessment of mechanical and 4D printing potential

Charles de Kergariou  1 , Graham J Day  2   3 , Adam W Perriman  3   4 , James P K Armstrong  5 , Fabrizio Scarpa  1
Affiliations

Flax fibre reinforced alginate poloxamer hydrogel: assessment of mechanical and 4D printing potential

Charles de Kergariou et al. Soft Matter. .

Erratum in

Abstract

The mechanical and printing performance of a new biomaterial, flax fibre-reinforced alginate-poloxamer based hydrogel, for load-bearing and 4D printing biomedical applications is described in this study. The-self suspendable ability of the material was evaluated by optimising the printing parameters and conducting a collapse test. 1% of the flax fibre weight fraction was sufficient to obtain an optimum hydrogel composite from a mechanical perspective. The collapse test showed that the addition of flax fibres allowed a consistent print without support over longer distances (8 and 10 mm) than the unreinforced hydrogel. The addition of 1% of flax fibres increased the viscosity by 39% and 129% at strain rates of 1 rad s-1 and 5 rad s-1, respectively, compared to the unreinforced hydrogel. The distributions of fibre size and orientation inside the material were also evaluated to identify the internal morphology of the material. The difference of coefficients of moisture expansion between the printing direction (1.29 × 10-1) and the transverse direction (6.03 × 10-1) showed potential for hygromorphic actuation in 4D printing. The actuation authority was demonstrated by printing a [0°; 90°] stacking sequence and rosette-like structures, which were then actuated using humidity gradients. Adding fibres to the hydrogel improved the repeatability of the actuation, while lowering the actuation authority from 0.11 mm-1 to 0.08 mm-1. Overall, this study highlighted the structural and actuation-related benefits of adding flax fibres to hydrogels.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Open literature data related to loading speed and Young's modulus (E1) of alginate-based hydrogels.
Fig. 2
Fig. 2. Production of the composite hydrogel. (a) Flax fibres (b) hydrogel chemicals (from left to right: sodium alginate; poloxamer 407; calcium chloride dihydrate). (c) SpeedMixer DAC 150.1 FVZ centrifuge used to mix the hydrogel and the flax fibres. (d) Cellink+ bio printer used to prepare the composite.
Fig. 3
Fig. 3. Stress–strain curves for the single fibre testing tests conducted with (a) dry fibres (b) wet fibres. (c) Distribution of Young's modulus.
Fig. 4
Fig. 4. Procedure to measure the properties of the flax fibres. (a) raw milled fibres. (b) Example of microscope image taken to measure the dimension of the fibres. The length here ranges between 1 and 50 μm. (c) Example of image taken by the IDM system FASEP machine to obtain the dimensions of the fibres if their length ranges between 50 μm and 300 μm. (d) Histogram of the distribution of the fibres lengths measured with the optical microscope. (e) Histogram of the distribution of the lengths for fibres dimensions measured with the FASEP machine.
Fig. 5
Fig. 5. Distribution and interpolation of: (a) length and (b) width. Distribution and interpolation of aspect ratio (c) large view (d) zoomed in view. (e) Interpolation coefficients for the fit function of the different dimensions.
Fig. 6
Fig. 6. Successful and failed prints for the different fibre weight fractions used to produce the hydrogel composites. The pressure was measured on the printer's monitoring display. The schematics present the different issues faced with inappropriate printing pressure. For too low pressure the print is discontinuous. On the other hand, too high pressure leads to over spread of the material and not precise geometry.
Fig. 7
Fig. 7. Filament fusion test results. (a) Geometric parameters for fusion of the printed filament. The schematic on the top right present the different parameters displayed. An exponential interpolation (f: x → a × eb×x) is provided on the graph. (b) Example of printed specimen for a fibre weight fraction of 0%. (c) Example of printed specimen for a fibre weight fraction of 1%. (d) Coefficients obtained for the exponential interpolation displayed in (a).
Fig. 8
Fig. 8. Annulus scaffold test specimens. (a) Top view of a annulus scaffold. (b) Side view of an annulus scaffold. (c) Statistical distribution of the dimension of the annulus scaffold.
Fig. 9
Fig. 9. Influence of the flax fibres reinforcement on the mechanical properties of the alginate-poloxamer hydrogel. (a) E1: initial stiffness [0; 10 000] με (b) E2 stiffness [50 000; 70 000] με (c) σ strength (d) ε strain at failure.
Fig. 10
Fig. 10. Post-processing of the images acquired via CT-scanning. Each grade of blue presents one section considered for measuring the porosity. (a) Specimen considered for CT-scanning. (b) Section 1, (c) Section 2, (d) Section 3, (e) Section 4, (f) Section 5 visualisation of the porosity in the different sections of the specimen presented in (a). The overall porosity measured in the specimen is 6.9 ± 1.1%.
Fig. 11
Fig. 11. (a) Influence of the amount of flax fibre on the area under the curve (AUC) measured. (b) Optical microscope image of failed specimen cross-section. Dark blue arrows pointing upwards show the fibres pull out during fracture. Dash light blue arrows pointing downwards indicate large voids on the surface of the specimen. Light blue arrows pointing downwards show the large voids split during fracture.
Fig. 12
Fig. 12. Crack branching mechanism. A main crack propagates through the material and branches when it encounters defects such as large voids or fibres in the vicinity of its path. The light blue arrow pointing upwards describes the main crack running through the specimen. The dark blue arrows pointing downwards indicate a non-catastrophic crack (called branch) from the main fracture crack. (a) Schematic. (b) Microscope Image.
Fig. 13
Fig. 13. Collapse test experiment. (a) Schematic of the test. (b) Photo of the experiment with the distance between two support block. (c) Collapse test, percentage of unbroken hydrogel for the different distances tested in the collapse test.
Fig. 14
Fig. 14. Viscosity with and without the flax fibres. (a) Strain rate versus complex viscosity for fibre and without the fibres. (b) Schematic of the viscosity shape.
Fig. 15
Fig. 15. Fibre orientation measurement. (a) Specimen used and cut made in it. (b) and (c) Schematics of the cut made on the specimen. (d) and (e) Example of SEM images perpendicular to the cuts made on the specimen. Fibre orientation. (f) Schematic showing the angles ξ and ϕ relative to the printing direction (g) distribution of fibre orientation along ξ angle (c) distribution of fibre orientation along ϕ angle.
Fig. 16
Fig. 16. 4D printing actuation of [0°; 90°] specimens. (a) and (b) Wet and dry specimen for 1% FWF, respectively. (c) and (d) Wet and dry specimen for 0% FWF, respectively. (e) Comparison of actuation for 1% and 0% FWF.
Fig. 17
Fig. 17. Rosette actuation with 1% flax fibre reinforced alginate-poloxamer hydrogel. (a) Wet specimens in cross-linking material after 24 h (b) dry specimens at room humidity. The blue arrows presents the highest point of the specimen after the actuation. A video in ESI, shows the full actuation of the structure.
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