3D printing of infill-based tpu auxetic metamaterials: a slicer-controlled design-to-fabricatıon experiment

Abstract

In recent years, designs disciplines across architecture, aerospace, product development, and wearable technology, have witnessed increasing interest in materials and structures capable of responding to external conditions through controlled flexibility, deformation, and recovery. Within this spectrum, auxetic metamaterials have emerged as a notable class of engineered systems defined by negative Poisson's ratio (NPR) behavior, in which lateral expansion occurs under tension rather than contraction. This counterintuitive response is ruled primarily by micro-architectural configuration, the arrangement of cells, hinges, joints, and voids, rather than by material composition alone. Given that, auxetics provide a geometry-driven path toward lightweight structures with tunable porosity, localized deformation, and repeatable mechanical behavior. In parallel, additive manufacturing, particularly fused deposition modeling (FDM), has enabled the accessible fabrication of such micro-architectures. This thesis investigates infill-based 3D printing as a design-to-fabrication medium for developing elastomeric auxetic metamaterials. Using rigid PLA as an initial evaluation material and TPU 95A as the primary elastomeric medium, the study frames slicer logic and toolpath decisions as an active layer of design control and decision-making, through which deformation tendencies, porosity, and material efficiency can be embedded directly into fabricated specimens. Following a focused literature review of metamaterials, auxetic deformation mechanisms, pattern-based design strategies, and additive manufacturing workflows, the research positions itself at a specific underexplored junction. While auxetic behavior has been widely examined through geometry and material selection, the systematic role of slicer-controlled infill strategies as a means of programming deformation and response remains inadequately explored. The thesis therefore investigates how infill pattern, density variation, and toolpath configuration can be treated as primary design variables influencing the observable behavior of 3D-printed auxetic metamaterials. Rather than approaching infill as a secondary structural filler, it is reframed as a micro-architectural mediator between digital geometry and physical performance, enabling the controlled production of lightweight, deformable, and recoverable TPU-based specimens. The study consists of an experiment-driven, prototype-based methodology structured into 3 sequential phases. It begins with exploratory pattern investigation, moving from analog and visual inspection to computational pattern generation and digital simulation. During this stage, auxetic pattern families are sorted in an exploratory prioritization list based on geometric clarity, deformation legibility, and frequency in literature, setting a structured design space rather than a performance ranking. Typologies of highest potential are translated into parametric definitions using visual programming, enabling calculated variation and comparative evaluation. Physical prototyping initially proceeds through PLA-based specimens to validate geometric articulation, print fidelity, and deformation logic independently of elastic material behavior. A material transition stage follows, using silicone to observe elastomeric response while revealing restrictions in controlling micro-scale architecture through inverse mold-based casting. The central experimental stage then shifts to TPU 95A, where infill-based fabrication becomes the primary mechanism for producing elastomeric auxetic metamaterials. Across prototypes, slicer-dependent parameters are systematically modified and documented to examine how fabrication decisions influence deformation clarity, porosity, elastic recovery, and structural resilience. Digital simulation supports comparative observation, while results are consolidated through a prototype documentation matrix that records typology, infill strategies, printing parameters, and observed behavior across all tested categories. The core contribution of the thesis lies in establishing a slicer-dependent design-to-fabrication workflow that links auxetic typology to infill-driven behavior in elastomeric 3D printing. Within this framework, deformation is seen as a programmable outcome of micro-architectural decisions, shaped through infill pattern selection, density variation, shell and wall configuration, extrusion settings, and toolpath logic. This approach enables rapid iteration without redesigning base geometry, as a single typology can express multiple behaviors through slicing strategies alone. Through comparative evaluation of non-auxetic categories and multiple auxetic categories, including sinusoidal, fidget, rotating, multidirectional, hierarchical, and functionally graded systems, the study reveals how infill logic directly impacts perceived adaptivity, deformation responsiveness, and elastic recovery. Rotating auxetic configurations consistently emerge as high-priority candidates due to their visible expansion mechanisms and reliable behavior in elastomeric 3D printing, while hierarchical and graded specimens expose how spatial variation in infill density can support programmable elasticity and confined performance tuning. Sustainability emerges as a parallel finding throughout the study. By relying on infill-based 3D printing rather than solid 3D printing, the prototypes achieve substantial material reduction, lower density, and reduced print time while retaining auxetic behavior. Comparative testing further revealed that while gyroid infill provides superior deformation continuity and isotropic response, 3D honeycomb infill surpasses gyroid in cost efficiency when used volumetrically, requiring less filament and shorter fabrication time. This observation underscores the potential of infill design as a sustainability measure and motivates future research into infill patterns optimized simultaneously for deformation performance and environmental efficiency. In the final stage, the thesis introduces pneumatic deformation as an experimental observation to examine how nominated auxetic specimens with highest exploratory priority respond under internal air pressure. This stage is viewed explicitly as qualitative rather than as a standardized engineering assessment. Inflation is used to track reversible deformation behavior, elastic recovery, and curvature tendencies emerging from the interaction between auxetic topology, void distribution, hinge behavior, and slicer-controlled micro-architecture. These observations verify the broader claim that planar auxetic textiles can be steered toward volumetric responses through controlled fabrication logic and material elasticity, without positioning pneumatic actuation as the primary force behind the research. Overall, the study positions infill-based TPU 3D printing as a viable, fabrication-aware approach for producing lightweight, breathable, and deformation-responsive auxetic metamaterials through a replicable and extendable experimental framework. By grounding the investigation in systematic prototyping, exploratory prioritization, and a structured documentation matrix, the research addresses a persistent gap in auxetic metamaterial studies, in which experimental outcomes are often presented in isolation and without a shared foundation suggesting proper comparison. The documentation matrix functions not only as a synthesis of results but as a handy methodological scaffold, enabling future researchers to reproduce, extend, or recalibrate the same parameters, such as infill logic, density gradients, shell configuration, and material selection, while maintaining continuity in evaluation. Beyond its immediate findings, the framework establishes a foundation for future investigations into elastomers of lower hardness, custom infill pattern design, and functionally graded micro-architectures optimized for both mechanical performance and material efficiency. The observed trade-offs between deformation clarity, structural stability, and filament usage point toward opportunities for integrating sustainable infill strategies, in which material reduction and print efficiency are treated as design objectives alongside mechanical behavior. Moreover, the slicer-dependent approach opens opportunities for the development of custom slicing tools or plugins that would allow designers to program deformation logic directly at the toolpath level, democratizing access to programmable metamaterial fabrication even more. In this sense, the thesis outcome extends beyond the specific prototypes produced and is not limited to the documentation address too as its only contribution, but rather offers a design-to-fabrication paradigm in which infill geometry is elevated from a secondary printing parameter to a primary architectural design medium, capable of encrypting motion, porosity, and elasticity directly into material systems. By doing so, the research supplies cumulative knowledge-building across different fields of design, and positions infill-based auxetic metamaterial fabrication as a fertile ground for future work at the intersection of computation, material behavior, and sustainable design.