Magnetostrictive Actuation of a Bone Loading Composite for Accelerated Tissue Formation

When bone is dynamically loaded it adapts its shape to better support the load. We have developed a magnetostrictive composite consisting of Terfenol-D particles encapsulated in an epoxy resin that changes length when exposed to magnetic fields. When bonded to the surface of a porcine tibia ex vitro, the composite produces surface strains greater than 900？με at a frequency of 30？Hz and magnetic field of 170？kA/m. This is more than sufficient strain magnitude and frequency to promote cortical bone growth in both rats and turkeys and to maintain cortical bone structure in humans. Key advantages of the composite over conventional electromechanical or thermomechanical actuators are its simplicity, compact size, and remote actuation. A mathematical model describing the strains and stresses in the bone is presented. 1. Introduction Bone structure continuously evolves by building new bone and resorbing old bone [1]. The dynamic property of bone remodeling allows bone tissue to adapt to changes in its loading environment [2]. Although the exact mechanism explaining bone remodeling remains elusive, interstitial fluid flow around a particular subset of bone cells called osteocytes is thought to be essential [3]. Eccentric loading of a long bone creates a bending moment and causes the bone to curve. This creates a region of tension on one side of the bone and a region of compression on the opposite side. As the load is cycled, interstitial fluid flows around osteocytes from regions of compression to regions of tension resulting in shear stress on the cell walls. Shear stress is thought to incite a molecular cascade resulting in new bone growth [4, 5]. In humans, strains below 200？με do not stimulate cortical bone remodeling [6]. Strain values between 200 and 2000？με represent physiological levels of strain on the human skeleton. Above 2000？με, the rate of bone formation exceeds the rate of bone resorption, and growth is observed [7]. Studies involving turkeys have shown that a 30？Hz, 100？με signal is sufficient to maintain bone mass [8], and studies involving rats have shown that a 2？Hz, 930？με signal is sufficient to produce bone growth [9]. Along with strain magnitude, the strain rate plays an important role in new bone formation [10]. Based on a compilation of previous studies where animal bones were subjected to mechanical loads in vivo, Turner [11] and Burr et al. [12] concluded that the strain rate and magnitude are related to the strain stimulus by where is the strain stimulus, is a proportionality constant, is the strain magnitude, and is the strain