Yuehao Luo
Graduate student
e-mail: yuehao@temple.edu
Parsaoran Hutapea
1
e-mail: hutapea@temple.edu
Department of Mechanical Engineering,
Temple University,
1947 N 12th Street,
Philadelphia, PA 19122
Stress-Strain Behavior of a Smart
Magnetostrictive Actuator for a
Bone Transport Device
The ultimate goal of our research is to develop a bone transport device using a magne-
tostrictive alloy actuation system. The device is designed to be subcutaneously mounted
on the periosteal surface of the tibia. The magnetomechanical behavior of Terfenol-D
smart magnetostrictive material has been well investigated in the literature when a mag-
netic field is applied along the longitudinal direction of the Terfenol-D material (perpen-
dicular to the material’s magnetic moment). However, the requirement of our device is to
have the magnetic field transversely applied on the Terfenol-D material (along the ma-
terial’s magnetic moment). Therefore, the objective of this work was to study the magne-
tomechanical behavior of Terfenol-D under a transversely applied magnetic field. Experi-
mental work was performed and a Terfenol-D material constitutive behavior was
investigated. DOI: 10.1115/1.2997331
Keywords: Terfenol-D, smart magnetostrictive material, ilizarov, fitbone
1 Introduction
The goal of our research is to develop a wireless, remotely
activated, and implantable bone transport device using a
Terfenol-D smart magnetostrictive material as an actuator. The
device is implanted near separated tibia bone fragments created
through distraction osteogenesis. The fundamental idea is to rejoin
the bone fragments and to elongate the regenerated bone tissues
with the aid of a proposed bone transport device 1–5.
Two commonly known bone transport methods, as illustrated in
Fig. 1, are the Ilizarov and Fitbone fixators 3,4. Ilizarov fixator
1,3 is an external fixator mounted outside of the human body,
Fig. 1a. This fixator requires the tibia to be cut off in order to
leave a small gap between the bone fragments. The separated
bones are then fixed with the Ilizarov fixator that penetrates
through the skin, muscles, and bones. Naturally, new bone tissues
will regenerate to fill the small gap. The fixator is then turned to
stretch the new bone tissues. In general, the required force to
elongate the bone tissues is approximately 200 N. The treatment
is typically performed three to four times a day for a total of
1 mm of lengthening per day. The lengthening process should not
be done too quickly so that it does not overstretch the tissues, or
too fast, which can result in the early solidification of the bone.
After the gap is closed, the patient continues to wear the frame
until the new bone solidifies. When the process is completed, the
device must be removed from the patient.
The Fitbone internal fixator 4 is an implantable bone trans-
port device Figure 1b. The device is made of stainless steel,
powered by an electric motor, and controlled by a wireless hand-
held transmitter. The fixator is made of a telescopic nail implanted
through an incision on the knee. After distraction osteogenesis is
performed, the telescopic nail is attached to the bone fragments
using screws. The lengthening process is performed using a wire-
less transmitter that communicates with an implanted antenna for
controlling the electric motor. The basic process is similar with the
Ilizarov method and when it is done, the device is removed from
the patient.
Both methods may be inappropriate for children. The Ilizarov
external fixator may be too heavy especially since the device has
to be worn for 3 to 6 months depending on the desired lengthen-
ing. In addition, Ilizarov method can be extremely painful, un-
comfortable, infection-prone, and it causes unsightly scars. The
major disadvantage of the Fitbone method for children is that it
requires drilling through the marrow cavity of the tibia that can
damage growth plates.
Our proposed device is an internal fixator based on the
Terfenol-D smart magnetostrictive material. The device uses a
gear and ratchet system and a titanium frame Fig. 2 and is sub-
cutaneously mounted to the periosteal surface of the tibia and
remotely activated. The basic concept of the proposed device is to
activate the Terfenol-D rod by means of a transverse magnetic
field so that the Terfenol-D rod elongates in the longitudinal ver-
tical direction. When the Terfenol-D elongates, it moves the gear
along with the spring in the vertical direction Fig. 2c. The
spring in the proposed design must have a stiffness the same as
the smallest stiffness of the Terfenol-D bar. The Terfenol-D stiff-
ness is controlled by the variation of transverse magnetic fields.
The ratchet holds the load that is required to stretch regenerated
bone tissues. When the magnetic field is removed, the Terfenol-D
rod shrinks to its original length causing the spring to carry the
load and to pull the lower bar that is mounted to the periosteal
surface of the tibia to stretch the bone tissues Figs. 2d and 2e.
The magnetic field can be varied by positioning the magnets to
control the gear and ratchet system. This process is repeated until
a desired lengthening of the bone tissues is completed.
Terfenol-D is a ferromagnetic material that is produced in au-
tomated crystal growth machines. It is an alloy of terbium, dys-
prosium, and iron metals and has the large room temperature mag-
netostriction. Its twin structure can be correlated with the
magnetic domain structure if the material has high magnetocrys-
talline anisotropy. Thus, an applied magnetic field can influence
the material’s twin structure and cause deformation by twin
boundary motion 6,7. The field-induced strain can reach ap-
proximately 250 s 6. Terfenol-D is a solid-state transducer ca-
pable of converting very high energy levels from one form to
another. In the case of an electrical-to-mechanical conversion, the
magnetostriction of the material generates strains twenty times
greater than traditional magnetostrictive materials, and two to five
times greater than traditional piezoceramics 6,7. Terfenol-D can
be fast activated by an applied magnetic field and is capable of
generating the required force for the bone transport procedure.
1
Corresponding author.
Manuscript received March 13, 2008; final manuscript received August 14, 2008;
published online October 23, 2008. Review conducted by Vijay Goel.
Journal of Medical Devices DECEMBER 2008, Vol. 2 / 041002-1 Copyright © 2008 by ASME
Downloaded 10 Nov 2008 to 155.247.51.214. Redistribution subject to ASCE license or copyright; see http://www.asme.org/terms/Terms_Use.cfm