Shape-morphing and shape-locking describe objects’ ability to transform their shape and stay in that state, respectively. These qualities often come into play when grabbing or supporting an object. The contact area is increased by changing the shape, after which sustaining the shape maintains the support over time. We can imagine several ways of implementing this in engineered structures. One way of implementing shape-morphing and -locking is using kinematic movement, where a structure comprises relatively rigid parts connected by joints that allow motion. This work focuses on this type and presents a framework for analyzing it.

We identified and studied the factors determining the performance of kinematic morphing and locking structures. Three distinct steps were employed that explore the envelope of possible designs by i) creating and verifying a morphing modeling approach based on multibody dynamics principles, ii) analyzing a morphing and locking structure by applying the model, designing and experimentation, and iii) analyzing the effects of design parameters on the morphing and locking qualities in the light of geometrical features such as curvature. Spanning these steps, we developed several structures and fabricated them with additive manufacturing. These structures are i) a 3D modular system that allows easy experimentation with layouts and joint types, ii) an essentially 2D system that deforms in-plane and is selectively and reversibly locked by applying magnetic fields, and iii) a 3D deforming non-assembly metallic structure that can follow single and double curvatures and is irreversibly locked as a whole by applying bone cement. To assess the performance of these structures, in addition to the presented simulation approach, we developed methods that test the physical morphing and locking qualities visually and mechanically. These methods included 3D-scanning and mechanical testing, accompanied by digital image correlation to measure full-field strain distribution.

The presented framework shows the potential of kinematic shapemorphing and -locking mechanisms to endow structures with new functionalities. Specifically, we showed that structures incorporating the mechanism can be rationally designed, manufactured, and assessed. A multibody model predicts the morphing behavior accurately. We can use such a method to design the structures for specific applications such as orthopedic implants or soft robotics. Locking the mechanism as a whole or per individual degree of freedom obstructed the morphing capability effectively, whether or not in a reversible manner. Simulations and experiments show that we can alter the transformation and load-bearing performance of the structures as desired by changing their design parameters. The matching capacities are affected by parameters like structural body shape, dimensional ratios, and the curvature of the to-be-attained shape. These principles can be further developed and tailored for specific needs in a variety of fields.

Biocompatible and shape-morphing metallic structures have been proposed for musculoskeletal applications to provide structural support to bony tissues. However, fabricating these structures to conform to a wide range of curvatures, including both single and double curvatures, remains a significant challenge. In this study, we present and analyze structures featuring a regular tiling network connected by spherical joints, forming a chain mail-like mechanism capable of adapting to complex geometries with clay-like flexibility. Simulations using a multibody kinematics model show that parameters such as unit cell shape, dimension ratios, and substrate curvature affect the shape-matching abilities of the structure. Experimental validation using specimens additively manufactured through laser-based powder bed fusion (from Ti6Al4V) and full-field strain measurements performed through digital image correlation confirms the simulation results, demonstrating that reducing structural density (i.e., fewer bodies, struts, and joints per unit area) improves shape adaptability. However, the improved shape morphing capability often comes at the expense of mechanical strength under uni-axial tensile loads. These findings provide a framework for optimizing structures designed to achieve efficient surface conformance and adaptability in load-bearing applications.

Acetabular defects pose significant challenges in orthopedic surgery, particularly in revision total hip arthroplasty (THA). Here, we design, additively manufacture, and evaluate shape-morphing porous implants with kinematic structures to address these defects. Three defect types were examined using synthetic hemipelvis models: posterior wall, cranial-posterior combination, and central-posterior defects. The implants were secured with screws and bone cement, and their surface conformity was assessed through micro computed tomography (µCT). Biomechanical performance was evaluated under quasi-static compression and cyclic loading conditions. Results demonstrated high surface conformity of the flexible mesh across all defect types, with minimal differences from healthy acetabula (< 10 mm). The mesh implants exhibited strong load-bearing capacity, with failures occurring only in the pubic region of the hemipelvis, while both the implants and mesh-cement interfaces remained intact. The implants withstood cyclic loading simulating half the body weight of a 80 kg patient for >1000,000 loading cycles with no evidence of fatigue failure, further confirming their durability. These findings suggest that the flexible mesh implant provides a potential solution for complex acetabular defects, offering anatomical conformity and mechanical stability, even in cases where conventional mesh grafts might be inadequate. Future studies, including cadaveric testing and clinical trials, are necessary to further validate these results in (pre-)clinical settings. Statement of significance: This study addresses the need for adaptable solutions to complex acetabular defects in revision total hip arthroplasty (THA). Traditional implants struggle to conform to severe bone loss and irregular geometries, risking suboptimal fit, and implant migration. We introduce a 3D-printed, shape-morphing porous implant with kinematic structures, offering high anatomical conformity, mechanical robustness, and support for bone graft integration. Combining the adaptability of patient-specific implants with the efficiency of standard designs, this implant reduces lead times while enabling a tailored fit. This innovative approach provides a reliable solution for managing complex defects, addressing limitations of conventional implants, and improving outcomes in orthopedic reconstruction.

Shape morphing is the ability of objects to adapt to different shapes and reduce stress concentrations through increased contact area. This is a common trait of natural and engineered objects and has several applications in, among others, soft robotics and orthopedic implants. Shape morphing is achieved through flexible materials or rigid components with either kinematic or compliant joints. An additional step, namely shape locking, is needed for sustained load support. Activation of a locking mechanism can be done with any energy, among which magnetism is one. Here, we present the implementation of a magnetic locking mechanism for kinematically deformable metamaterial structures that maintain shape and support loads upon locking. The structure consists of 3D printed rigid magnetic and non-magnetic components connected by hinges. We created several prototypes of the proposed designs using two additive manufacturing methods (i.e., material extrusion and multi-material jetting) and demonstrated its application in a closed-loop grid for arbitrary shapes. Moreover, we characterized the performance of the prototypes using mechanical tests and multibody kinematic system simulations. This work highlights the viability of the locking concept and provides design considerations for future applications. Further improvement and optimizations are needed for increased efficiency and effectiveness.

Shape-morphing structures have the ability to adapt to various target shapes, offering significant advantages for many applications. However, predicting their behavior presents challenges. Here, we present a method to assess the shape-matching behavior of shape-morphing structures using a multibody systems approach wherein the structure is represented by a collection of nodes and their associated constraints. This representation preserves the kinematic properties of the original structure while allowing for a rigorous treatment of the shape-morphing behavior of the underlying metamaterial. We assessed the utility of the proposed method by applying it to a wide range of 2D/3D sample shape-morphing structures. A modular system of joints and links was also 3D printed for the experimental realization of the systems under study. Both our simulations and the experiments confirmed the ability of the presented technique to capture the true shape-morphing behavior of complex shape-morphing metamaterials.