Driving macro-scale transformations in three-dimensional-printed biopolymers through controlled induction of molecular anisotropy at the nanoscale

Abstract

Motivated by the need to harness the properties of renewable and biodegradable polymers for the design and manufacturing of multi-scale structures with complex geometries, we have employed our additive manufacturing platform that leverages molecular self-assembly for the production of metre-scale structures characterized by complex geometries and heterogeneous material composition. As a precursor material, we used chitosan, a chemically modified form of chitin, an abundant and sustainable structural polysaccharide. We demonstrate the ability to control concentration-dependent crystallization as well as the induction of the preferred orientation of the polymer chains through the combination of extrusion-based robotic fabrication and directional toolpathing. Anisotropy is demonstrated and assessed through high-resolution micro-X-ray diffraction in conjunction with finite element simulations. Using this approach, we can leverage controlled and user-defined small-scale propagation of residual stresses to induce large-scale folding of the resulting structures.

Keywords: additive manufacturing; biological materials; large-scale structures; molecular anisotropy; multiscale design; self-folding.

Figure 1.

Overview of the WDFP and specific examples of manufacturable geometries. (a) INSPIRATION: biomimetic case study digital design process for 2.5-dimensional toolpath generation based on dragonfly wing and leaf venation patterns of a 3-cm-long dragonfly wing specimen and a 10 cm-long dried leaf specimen. (a, right) VIRTUAL: 3 m-long digitally generated toolpath with hierarchical geometric patterning. (b) PHYSICAL: Fabrication platform features: robotic arm positioning system with a 1 m reach and 10 kg payload and multi-nozzle pressure deposition system carrying diverse chitosan hydrogel concentration ranging from 2% to 12% w/v in 4% acetic acid. (c) RESULTS: 3 m-long self-folded column cured at ambient conditions and a magnified view of structural folding along toolpaths induced through the fabrication process.

Figure 2.

Material formulation and molecular-scale crystallographic characterization (a) Pictorial representation of our material preparation strategy: through the solubilization of different concentrations of raw chitosan powder in acetic acid, we can create a wide range of material viscosities that can be extruded for the production of complex 2.5-dimensional architectures (cf. figure 1). (d) X-ray diffraction pattern (XRD) (Rigaku Miniflex) of raw unprocessed chitosan. (e) XRD pattern (Rigaku Miniflex) of chitosan after solubilization in AcOH, printing and drying. The solid blue pattern represents the Pawley fit of an α-chitosan structure, which was subsequently used to generate the proposed crystal structure of α-chitosan shown in (b–g). (f) Powder XRD (Argonne APS 11BM) analysis of dried acetic acid-solubilized chitosan films (of equal mass), demonstrating a clear correlation between polymer concentration (2–12%) and the extent of crystallinity. The background diffraction pattern obtained from the Kapton capillary sample holder is highlighted in grey.

Figure 3.

Directional anisotropy of 2.5-dimensional-printed chitosan-based constructs. (a) Photograph of a 2.5-dimensional-printed chitosan ribbed structure with an inset diffraction pattern corresponding to the cast 2% chitosan film. (e) X-ray transmission intensity plot used for identification of the regions of interest at the synchrotron beamline (from red to green to blue, the colour scale corresponds to relative sample thickness). Left rib C: 12% w/v chitosan in 4% w/v acetic acid, 2 mm inner diameter nozzle and two layers in height and one in width; Right rib D: 12% w/v chitosan in 4% w/v acetic acid, 2 mm inner diameter nozzle and three layers in height and six in width (parallel extrusion). (b,d) XRD patterns acquired from ribs C and D in panel (e). Images are scaled yellow at the most intense reflections. Green dashed lines indicate the azimuthal direction of maximum diffraction intensity. For both ribs, this line is parallel to the rib direction. (c,g) Diffraction patterns of (b,f) shown with azimuthal chi variable ‘unrolled’ along the horizontal axis. (d,h) Intensities (b,f) integrated within a–q interval around the intense reflection and plotted versus chi. The data reveal that the preferred orientation of the chitosan crystallites follows the direction of each rib.

Figure 4.

Numerical simulations and experiments of fabrication-induced folding as a function of structural anisotropy. Top: contour plot showing the dependency of the folding behaviour on the direction of anisotropy θ, and the size of anisotropy introduced by varying the swelling ratio parallel and orthogonal to θ, ${\displaystyle \left(\frac{S\mathrm{\perp }\theta }{S||\theta }\right)}$ . The folding direction is characterized by the absolute change in distance between the outer points lying on the short axis of the square. Bottom: experimental verification of the numerical model, showing a relatively high level of anisotropy (i.e. $\frac{y-x}{L}$ ≥ $\frac{\surd 2}{L2}$ ), as evidenced by the direct link between print direction and folding direction (noted with arrows on two- and three-dimensional schematics and on photographs of shaped constructs).

Figure 5.

Schematic of the multi-scale behaviour model of our material system and fabrication strategy, where the induced orientation of nanoscale chitosan crystallites ultimately drives macro-scale folding of the final additively manufactured form. (a) Chitosan monomer, (b) acetic acid-solubilized chitosan crystallites depicted following a structural rib orientation printed with the WDFP, (c) 2.5-dimensional-printed structural rib and (d) chitosan construct folded along the print direction.