Additive manufacturing of cellulose-based materials with continuous, multidirectional stiffness gradients

Functionally graded materials (FGM) allow diverse applications in multidisciplinary fields from biomedicine to architecture. However, their fabrication can be tedious relative to gradient continuity, interfacial bending and directional freedom. Most commercial design software does not include property gradient data—hindering the exploration of design space suited for FGMs. In a new report on Science Advances, Pedro A.G.S. Giachini and a research team in architecture and urban planning, physical intelligence and medicine, in the U.S., Germany and Turkey designed a combined approach of materials engineering and digital processing. The method facilitated extrusion-based multimaterial, additive manufacturing of cellulose-based, tunable viscoelastic materials.

The constructs maintained continuous, high-contrast and multidimensional stiffness gradients. Giachini et al. established a method to engineer sets of cellulose-based materials with similar compositions, yet with distinct mechanical and rheological properties. The team also parallelly developed a digital workflow to embed gradient information into design models with integrated fabrication path planning. The team combined the physical and digital tools to achieve similar stiffness gradients through multiple pathways to achieve open design possibilities that were previously limited to rigid coupling of material and geometry.

Functionally graded materials (FGMs) can gradually change composition or structure in a continuous, stepwise manner to give rise to changing properties of a composite. Principles of material design are similar to many naturally occurring substrates, built to fulfil multiple, at times conflicting design requirements in diverse fields including thin film coatings, biomedical engineering and architecture. The FGMs can better distribute stress at interfaces, program deformation of soft actuators and influence the speed of cell migration.

Giachini et al. combined materials engineering and digital processing as an FGM manufacturing method for constructive and mass transport processes to create continuous gradients. They accomplished this by engineering solutions of a cellulose derivative to provide tunable viscoelastic properties with controlled extrusion, while using the digital workflow to embed gradient information into the designs and generate a custom G-code to control the operational system [three-dimensional (3-D) printer and syringe pumps]. The team used filaments of varying compositions and cross-sections to facilitate molecular diffusions across filament boundaries and create continuous gradients. They highlighted the relevance of combining materials engineering with custom manufacturing technologies and an environmentally friendly and abundant biopolymer-based fabrication material. By designing such physical and digital tools, the team will be able to create multidimensional and continuous stiffness gradients via a variety of methods to expand design possibilities for FGMs.

Giachini et al. selected hydroxethyl cellulose (HEC); a thickening and gelling derivative of cellulose as the base material, due to its non-toxic, biodegradable and environmentally friendly constitution. The gelation point of HEC occurred at 96 minutes, transitioning from an aqueous solution to a solid hydrogel. The scientists optimized the solution parameters to minimize the rate of solution viscosity. When they added citric acid (CA) to the solution, the gelation rate slowed down the most for satisfactory extrusion consistency. The team then characterized the printed material to understand the effect of additives, where the addition of lignin significantly increased the stiffness and tensile strength, while the inclusion of CA decreased these mechanical properties. The combined lignin- and CA-differentiated solutions provided a variety of mechanical properties to print objects with property gradients. The team then noted a decrease in stiffness and increase in size and weight of printed samples with increasing relative humidity, which they explored for applications involving shape-changing structures.

During the design-to-fabrication workflow, the team combined geometric models with gradient data to create FGM data and generate a fabrication code. As a platform for this workflow, they used Grasshopper; a visual programming interface embedded within the 3-D modelling software Rhinoceros 3-D. The team varied the fabrication parameters to create the graded objects of interest by superimposing layers, varying the amount of the material and its composition.

The fluidity of materials with lower viscosity provided object continuity, while more viscous mixtures discretely altered the stiffness. The diffusion between contrasting materials guaranteed interlayer continuity to create continuous and pliable sheets of material with patterned reinforcements. The deposition rate depended on the extrusion rate of the syringe pumps and the speed of the printer's nozzle. Giachini et al. embedded these fabrication parameters in the geometric data and translated the data into fabrication commands to coordinate the distribution of material, explore material flow and allow equal deposition paths to manufacture objects with varied geometric stiffness.

They designed data of mixing ratios, for translation into fabrication codes that modified the extrusion rate of the syringe pumps and developed a computational strategy to optimize the path of deposition to address the challenges of the setup. The sample fabricated using the gradient-optimized path showed higher material contrast immediately after deposition. The team tuned the gradients at the local and global scales using the developed strategies. They tuned the local stiffness according to the material's Young's modulus to control material distribution and influence object deformation. For example, Giachini et al. submitted the materials to external forces to achieve distinct deformation behaviors by distributing stiffness along specific directions or patterns.

The approach of using external force to generate the final shape of an initially flat object will allow designers to leverage simplified 2-D manufacturing strategies and avoid complex 3-D processes. The method will have applications in industrial product designs, architectural design systems that explore elastic bending of planar objects to achieve form and structural integrity and in developing compliant mechanisms and soft robotics. The team validated their experimental observations using a simulation, which mirrored the physical prototype, providing feedback on the distribution of stresses in the deformed sample.

In this way, Pedro A.G.S. Giachini and colleagues combined materials engineering and digital processing to control material mixing and deposition to extrude tunable, viscoelastic materials with continuous, high-contrast and multidirectional stiffness gradients. They established a method to engineer a base solution into a catalog of fluidic cellulose-based materials containing district mechanical and rheological properties to provide a physical foundation for stiffness gradients. The flexibility of the method allowed the team to adapt scalable and adaptable processes that can be applied to a variety of gradient fabrication processes. The developed method will be further optimized to overcome limitations and push the existing potential to print 2-D or 2.5-D objects and create fully-formed 3-D objects with internal functional property gradients.