BioSpring™:  Promoting Orthopedic Healing Through Small-Scale Spring Implantables

Abstract

The BioSpringTM offers a millimeter-scale, biocompatible spring with high compressibility and load-bearing capacity, enhancing the linear range of motion and providing consistent passive force to promote bone growth in the context of numerous orthopedic implants. This unique double-walled polymeric design, combining a shape-stabilizing outer mesh with an elastic inner material, enables structural elasticity in one desired axial direction without obstructive diametric deformation.

Most current orthopedic implants rely on fully rigid fixation, making them non-ideal for natural longitudinal fluctuations of growth plates, particularly in pediatric patients. This rigidity often results in stunted or uneven bone growth and poorer recovery outcomes. Furthermore, rigid implants face challenges in biomechanical compatibility with soft tissues within joint spaces. Existing metal springs provide adequate resistive force but allow too little vertical deformation, while their production costs and degradation risks are high. Polymer-based springs, meanwhile, lack standardization and fail to withstand enough load cycles for long-term use.

The BioSpringTM design specifications focus on dimensions, mechanical properties, and biocompatibility. The spring will be sub-centimeter in scale, with a 1:2 solid-to-free length ratio and minimal diametric deformation under compression. It will provide an approximately linear resistive force up to 35 N, durable for several years under continuous stress. Key material properties include Poisson’s ratio, Young’s modulus, and degradation rates.

For prototyping, Fused Deposition Modeling (FDM) with thermoplastic polyurethane (TPU) filaments will shape the outer geometry, while polydimethylsiloxane (PDMS) casting will form the inner elastic core, allowing rapid, cost-effective iteration in-house. Once prototypes meet design criteria, BioSpringTM will be manufactured via injection molding with medical-grade materials.

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