Accelerated Bone Growth Remotely Induced by Magnetic Fields and Smart Materials

Abstract

This paper presents the novel approach of employing active or “smart” materials to serve as mechanical stimuli for promoting bone growth. It has been shown that when bone is placed under oscillating bending stresses, it will adapt to better support the load and as a result, the bone will grow and strengthen. Furthermore, the strain stimulus induced in bone as a result of oscillating bending stresses is directly proportional to both the magnitude and frequency of the strain signal. Most current research is based on electromechanical or thermomechanical methods to produce bone stress. The resulting systems can be prohibitively large or require frequent surgery for adjustment. This proposal is focused on the creation of a smart materials-based bone-loading apparatus that could produce enough stress to promote bone growth while remaining compact and minimally invasive. To satisfy these requirements, magnetostrictive compounds that deform in the presence of magnetic fields are considered. These materials have the ability to convert magnetic energy to mechanical energy and vise versa. Applied magnetic fields cause domains to rotate thus changing the overall shape of the material. One magnetostrictive material considered is Terfenol-D, an alloy of terbium, dysprosium, and iron. This alloy was chosen because it exhibits a high saturation strain, or maximum strain level attainable, relative to other magnetostrictive materials. To increase functionality and resilience of the alloy, Terfenol-D is used in composite form using micron-sized Terfenol-D particles embedded in an epoxy matrix magnetically aligned during the cure of the composite. To magnetically activate the sample, a solenoid was constructed with 2400 turns of 20 AWG magnet wire able to produce a maximum quasi-static field of 405 kA/m. Initial quasi-static tests of the composite were conducted with free-free boundary conditions to measure the maximum magnetostriction of the sample. Strain was measured by two strain gages, one on each of the axial surfaces of a half-cylinder composite sample of Terfenol-D. Tests were conducted by applying a 180V sine wave signal at a frequency of 0.15Hz to the solenoid. The maximum strain level produced by the sample was 575 microstrain at a maximum field of 300 kA/m. Tests at 30Hz were then conducted producing 2300 microstrain at a field of 170kA/m. The Terfenol-D composite was then bonded to the surface of a porcine tibia. The composite was driven at a frequency of 30 Hz and a field of 170kA/m. Test results showed that the strain production on the surface of the bone exceeded 1000 microstrain. This is sufficient strain magnitude and frequency to promote cortical bone growth in both rats and turkeys, and maintain cortical bone structure in humans.