Origami-inspired Expandable Soft Cages for Interbody Spinal Fusion

Biomedical Devices and Instrumentation; Engineering Mechanics; Manufacturing

This research will explore the state-of-art design of origami-inspired, locally soft yet globally stiff, expandable biomedical cages for interbody fusion in spinal surgery.

Clinical significance: The most recent Global Burden of Disease identified back and leg pain as the most common cause of disability worldwide. Spinal disorders are considered as major causes. The mainstay treatments for spinal diseases are surgical procedures. Spinal fusion is such a predominant procedure. Interbody spinal Fusion is commonly performed (352,000 /year in the US) to treat many disorders of spine including degeneration, deformity, trauma, tumor, or infection, where removal of the damaged anatomical structure is required. Success of spinal fusion relies on the critical integration between vertebrae native bone and bone graft. The rate of pseudo-arthrosis is reported to be 10-40%. 

Problems of current cage and the proposed research: Over the last few decades, various spacers by structural design have evolved radically from its earliest rigid static rods to screw-based expandable cage forms, along with material choices from stiff alloys to polymers. However, most existing cage designs is challenged with conformal matching with bone structures in the areas near the cage because of predefined configurations and geometries of cages such as endplate angle and height of cage, which leads to micro-motion of segments subjected to physiologic forces and moments during daily activities and restrains with the biologic process of bone healing. Besides, for the uneven surface such as severe degenerative spine with endplates erosion, the existing cages cannot be conformably, if not at all, match with proximal and distal endplates. The non-conformal matching such as local gaps will render non-uniform distribution of forces and thus decreases the likelihood of local mechanical support and bony ingrowth, and subsequently causes cage subsidence.

In fundamental, the cage by structural design is expected to be expandable, adaptive for easy and noninvasive implantation, and once inserted, it could fully fill the disc/vertebrae space to provide sufficient mechanical support and stability. Uniform load distribution across the cage-bone interaction and close contact between cage and bone will promote bony ingrowth and prevent subsidence. Origami that originates from the art of paper folding could transform a flat square sheet of paper into a spatial three-dimensional sculpture. Inspired by origami, smart structures that can change their geometric shapes and morphologies in response to external surroundings and stimuli such as pressure, temperature and electric field have been used in engineering including origami-inspired deployable solar panels in spacecraft, origami-folded deployable hinges, origami-based morphing wings. Given the flexibility of origami structures and composition materials, exploiting its intelligent design is expected to address the current limitations of interbody fusion cage design.

In this research, we propose to explore the state-of-art design of expandable soft cages for spinal fusion by origami-inspired intelligent design through a multifaceted team of collaborators. The cage will be origami structures composed of soft materials. The hypothesis is that the soft materials will enable seamless local contact with near bones by large deformations, whereas the origami structure will enable overall high stiffness of the cage to mechanically support loadings by harnessing stitching and buckling deformation mechanisms of origami planes. We will investigate the fundamental mechanism of controllable expansion and multiple stable statuses in origami structures under a mechanical loading, and establish a conceptual design principle of an origami-based cage capable of being actuated by pneumatic pressure. This proposed work will offer a drastically new design that has never been explored before and represents a scientific and technological breakthrough in expandable cages for interbody spinal fusion.

Research objectives: In the proposed conceptual design of expandable soft cages, the expansion from the initially nearly two-dimensional flat to a controllable stable height state will be actuated by a pneumatic pressure through the integrated microchannels in structures. At each stable stage, the structure will be capable of sustaining a mechanical compression which mimics a pressure loading from surrounding bones in spinal fusion. We will plan to achieve the proposed research goal through the following three tasks:

 I.   To conduct a fundamental mechanism study on a controllable expansion of origami-structures with the focus on the design principle of multiple stable stages by investigating the local stitching and buckling deformation of planes under a mechanical loading.(Xu and Cai)

II.   To develop 3D printable soft materials and manufacture the designed origami-structures using 3D printing techniques. Soft polydimethylsiloxane (PDMS) with controllable flexibility in deformation will be programmed and printed through 3D printing.(Cai and Xu)

III.   To demonstrate the expandable performance of the manufactured origami-cages under a pneumatic pressure and its implantation to bone structures in rabbits. The focus will be on the continuous stable stages and overall stiffness of structures in mechanical testing under a certain external weight and in rabbits after implantation with a bone environment. (Li and Xu)

Desired outcomes

Thisproject will initiate a completely new research direction in soft, expandablemedical devices by tailoring engineering and medicine expertise, and bringbreakthrough engineering solutions to healthcare society. This project willalso help build a multidisciplinary research team across mechanicalengineering, materials science, biomedical engineering and surgery to pursueexternal research funding from NIH-NIBIB and NSF-CMMI.