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Device Technologies and Biomedical Robotics

Device Technologies and Biomedical Robotics - Poster Session C

Poster I18 - Mechanical characterization of an origami-inspired multistable tube for reconfigurable implants

Friday, October 25, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Brianna Leung

Bioengineering Undergraduate Student
University of Pennsylvania
Philadelphia, Pennsylvania, United States

Co-Author(s)

Gabriel Unger, MSE (he/him/his)

Ph.D. Student
University of Pennsylvania, United States
SE

Saúl R. Escorza

Mechanical Engineering Undergraduate Student
University of Pennsylvania
Philadelphia, Pennsylvania, United States
JC

Jonathan Chen, MD

Chief of Division of Cardiothoracic Surgery
Penn Medicine, United States
MF

Mark Fogel, MD

Director of Cardiac MRI and Cardiac MRI Research
CHOP, United States

Primary Investigator(s)

CS

Cynthia Sung, PhD

Associate Professor
University of Pennsylvania, United States

Introduction: The significant physical changes that pediatric patients undergo make effective implant development an ongoing clinical challenge. Most commercially available implants fail to accommodate growth, leading to reduced efficacy and potential complications. This inadequacy results in frequent reoperations, imposing substantial physical and financial burdens, especially for high-risk conditions like congenital heart diseases. For modulating blood flow in such cases, a remotely reconfigurable implant with a variable diameter within a constrained volume would be advantageous, and given the slow progression of growth and solely periodic adjustments, it would be energetically advantageous to utilize a non-powered structure. Multistable structures, which maintain configurations passively and require power only for transitions, are ideal. However, the actuation force needed to change between positions must be carefully tuned to enable selective actuation.

We explore the novel application of a multistable origami structure in pediatric flow-modulating implants. The State-Switching Reconfigurable Expanding Bistable Origami (SREBO) pattern, an origami bellows pattern with multiple stable states, is a promising candidate. To fulfill the need for remote reconfiguration, we control the SREBO using magnetic panels for actuation, an approach that has been used successfully on other patterns. The addition of folds and stable states in the SREBO pattern can affect the force needed to change between positions. To guide implant design, we examine the force response of vertical compression of the SREBO after varying geometric, material, and scale parameters. From this, we find consistently increasing forces in thickness and size scaling, but variable forces as we change stable state geometries.

Materials and

Methods: A geometric analysis of a compressed SREBO can identify the relationships between the stable states, associated heights, and inner diameters. This model relates the inner and outer radius based on pattern parameters: side length a, panel height h, state-switch fold angle α, and the number of sides n (Figure 1a). Additionally, it derives the relationship between the inner radius, outer radius, and the overall height of the structure (Figure 1b). These equations were used to tune parameters for target stable heights and diameters. The patterns were fabricated from PET sheets cut and perforated on a laser cutter, then manually folded. The SREBOs were finalized by joining an overlapping tab, and then adhering top and bottom reinforcing panels.

Average force peaks of either single- or multi-state SREBOs (a = 8.17mm, h = 5.4, alpha = 48.55°, n = 4) were measured while varying several factors: PET thickness (0.001”, 0.002”, 0.003”), size scale (2x, 2.5x, 3x, 4x, 5x, 6x), and the α angles of the state-switch fold (30°, 40°, 45°, 55°, 60°, 65°). For each trial, the model was secured to a 3D-printed test holder and then mounted to an MTS Electromechanical Test Machine (Figure 1c). During each trial, models were compressed to the theoretical maximum switch-state compression. Maximum peak force and cross-head location were recorded, demonstrating the force needed to switch states. The data was processed to find the average peak force and standard deviation.

Results, Conclusions, and Discussions: For single-state SREBOs, we observed that thickness and size scaling presented relatively consistent force patterns, while geometric manipulations yielded variable changes. Increasing thickness led to continually rising average force peaks (Figure 2a). Varying size scales from 2x to 5x showed a downward trend in average force peaks (Figure 2b). The 6x scale deviates from the pattern of decreasing force peaks, but this may be attributed to an outlier peak of 6.01N during the first trial. Varying alpha angles yielded the highest forces between 45 and 60 degrees, declining at higher and lower angles (Figure 2c). No peaks were observed at 30 degrees, which suggests force-loading being distributed at low angles and a switch-point of multistability based on manufacturing methods and geometries.

To characterize the impact of overlaid folds, pattern variations with a 46° state-switch fold ("Alpha-1"), a 64° state-switch fold (“Alpha-2”), and the overlaid combination of both folds ("Alpha-1-2") were tested (Figure 3a). Paired force peaks and force dips are present in Alpha-1-2 data, corresponding with actuation forces and stable states, respectively. Normalized force ratios are the peak force of an Alpha-1-2 model (averaged across all trials) divided by its single-fold counterparts. Alpha-1 normalized force ratios are consistently greater than 1, indicating that multi-fold models increased the average force peak value of the Alpha-1 fold (Figure 3b). However, there Alpha-2 normalized force ratios varied. This suggests that adding new stable states increases the force required to switch stable states, dependent on the angle of the state-switch fold.

This study demonstrates the design and construction of a multistable implant candidate and the impact of SREBO pattern manipulations on actuation force. Consistency in thickness or size scale changes are encouraging indicators that multistability can be reliably predicted when adjusted for different implant applications. These force trends suggest that actuation force can be adjusted to ensure selective actuation. The variable behaviors with different fold angles or overlaying folds indicate a need for further investigation into the interplay between stable state actuation forces and pattern geometry. Pursuing a comprehensive model will enable more accurate control of multistable SREBO structures for reconfigurable implant applications.

Acknowledgements (Optional): This work was supported in part by the Johnson & Johnson WiSTEM2D program, by the Penn Health-Tech program, and by the Penn Center for Undergraduate Research and Fellowships. We also thank Alec Lanter for his thoughtful discussion contributing to the premise of this work.