Session: 02-02 Shape Memory Alloy and Polymer Applications

Paper Number: 171974

171974 - Towards Moldable Applications Using Shape Memory Polymer-Based Auxetic Structures 

Adaptive moldable structures that can passively conform to varying shapes, rigidly lock into those configurations, and reset to a flat state on demand hold broad potential across many fields, particularly in mobility and medical technologies, where reconfigurability is essential to accommodate diverse and evolving user needs. Previous work has explored pneumatically activated systems, such as granular jamming and layer jamming, to create moldable structures. While granular jamming can achieve complex shapes, it typically results in bulky and heavy systems. Traditional layer jamming using inextensible sheets enables thinner, lighter structures but struggles to conform to double-curved surfaces without creasing, limiting geometric versatility. Beyond these pneumatic approaches, researchers have also explored Kirigami-inspired structures and auxetic metamaterials that enable biaxial in-plane deformation. These approaches improve surface conformability by allowing expansion and compression in both directions. However, most existing examples are either made from low-stiffness, elastically extensible materials like rubber or thermoplastic polyurethane, making them easy to reshape but unsuitable for load-bearing, or from high-stiffness, inextensible materials like sheet metal, which are structurally robust but difficult to reshape. This work addresses the challenge of designing a flat and lightweight structure that can be repeatedly molded into a wide variety of complex geometries, return to its original flat state, and be reshaped into new configurations on demand. The system architecture consists of a polyurethane-based shape memory polymer (SMP) layer that is made of periodically tiled auxetic unit cells, multiple two-way stretchable joule-heating layers, and thin elastomer layers that encapsulate the entire layup. The design of each unit cell is based on rotating polygonal structures, where the centers of each adjacent unit cell pairs are connected to one another via ligaments. This configuration enables biaxial expansion or contraction in response to uniaxial in-plane tensile or compressive forces. When heated above its glass transition temperature (Tg), the auxetic SMP structure softens and becomes easily deformable. As it is compressed against a target surface, adjacent groups of unit cells expand or contract locally in-plane to conform to the geometry. Cooling below Tg restores the structure’s stiffness, locking it into a rigid, load-bearing configuration. Reheating above Tg activates the shape memory effect, returning the structure to its original flat state and enabling remolding. The moldability performance of SMP-based auxetic structures is evaluated based on two key criteria: the ability to conform to a range of distinct target geometries and the resulting load-bearing capacity. A preliminary design of experiments is conducted to investigate how variations in unit cell geometry, specifically (1) rotation angle, (2) scale, (3) ligament width, and (4) unit cell height, affect overall moldability. These effects are experimentally characterized using a set of fabricated prototypes. The findings offer both design insights and quantitative performance data to inform the development of novel adaptive applications utilizing moldable auxetic SMP structures.

Presenting Author: Koray Benli University of Michigan Ann Arbor

Presenting Author Biography: Koray Benli received his B.S. and M.S. degrees in Industrial Design from Middle East Technical University, Turkey, and earned his Ph.D. in Design Science from the University of Michigan. He is currently a postdoctoral researcher in the Smart Materials and Structures Design Lab within the Department of Mechanical Engineering at the University of Michigan and the Toyota Research Institute of North America. His research focuses on developing next-generation mobility applications through the integration of constrained layer pneumatic systems and a range of smart materials.