Applications of Shape Memory Materials in Orthopedics
The Metal Physics Division at CAB has extensive experience in shape memory materials (SMAs). These materials exhibit particular mechanical properties that make them very interesting for applications in orthopedics and medicine, as illustrated below with some examples of the topics being researched.
What happens when a mechanical load is applied to an SMA?
Consider, for example, what happens when a deformation \(\Delta l\) is imposed on any (one-dimensional) body. As the applied deformation increases, so does the force $F$ required to sustain it. In a typical engineering material, this relationship is linear and limited by its plastic deformation. Shape memory materials exhibit a special behavior: once a certain level of deformation (or applied load) is reached, the deformation can continue up to about 8% of the initial length of the body without a significant increase in the applied load (we say the behavior is pseudoelastic). If we were to reduce the applied deformation, we would observe something similar in the opposite direction, as shown in the following figure:
What is the purpose of this?
This property is of interest for applications where it is necessary to maintain stress sustained over time on points that move appreciably. Consider, for example, the case of a foot deformity in equinovarus:
This deformity is characterized by an extension deviation (blue arrow), accompanied by a varus deviation (red arrow), often accompanied by supination (green arrow). The potential of shape memory alloys to treat such problems lies in the ability to make an orthosis capable of exerting approximately constant corrective stress, maintaining the applied load as correction progresses, without the need for adjustments, as with conventional materials, as illustrated below:
The division works on solutions for this and other similar problems, such as:
- Fallen arch syndrome
- Valgus foot
- Varus foot
- Genu varum - Genu valgum
- Equinovarus foot
What if we now increase the temperature of the alloy?
If we were to increase the temperature of an SMA that has been deformed to enter the pseudoelastic regime, without varying its deformation, we would observe that the load required to sustain it increases. This heating could be done, for example, by Joule effect, by passing an electric current through it. This is schematically shown in the following figure, where the load moves from point (1) to point (2):
And with this?
The effect described above shows one of the greatest potentialities of SMAs. With wires or strips of these materials, it is possible to make very compact actuators that allow applying controlled stress on a limb or a prosthesis to assist in the movement of a body part, for example, a hand:
The principle illustrated here is also being studied for the development of active orthoses to assist in walking.
Ongoing collaborative work:
- 2019-2020: Development of a passive orthosis using NiTi wires to improve the neurological rehabilitation process of patients with sequelae of strokes.
- 2017-2020: Work to optimize the following solutions based on SMAs:
- AFO for a 4-year-old patient with equinovarus foot.
- AFO for a 2-year-old patient with equinovarus foot.
- FO for a 13-year-old patient with varus foot.
- Passive knee orthosis, without articulation possibility, for a 5-year-old patient with congenital achondroplasia and genu varum.
- Passive knee orthosis, with flexion possibility, for a 12-year-old patient with genu valgum.
Working on the topic
Related publications Associated techniques and equipment Work on the topic!**
- Undergraduate theses:
- 2019 – 2020: Integrative Engineering Project by Rolando Cabrera, Mechanical Engineering, Balseiro Institute. Title: Development and optimization of an active orthosis for neurological rehabilitation of the lower limb. Supervisor: G. Alonso, Co-supervisor: J. Delmastro.
- 2018 – 2019: Integrative Engineering Project by Santiago Price, Mechanical Engineering, Balseiro Institute. Title: Application of shape memory materials to bioinspired systems. Supervisor: A. Yawny, Co-supervisor: H. Soul.
- 2018 – 2019: Integrative Engineering Project by Matías Mosquera, Mechanical Engineering, Balseiro Institute. Title: Design and construction of a knee orthosis to correct congenital malformations. Supervisor: G. Bertolino, Co-supervisor: G. Alonso.
- 2015 – 2016: Integrative Engineering Project by Alex Lövi, Mechanical Engineering, Balseiro Institute. Title: Stress distribution analysis associated with the use of superelastic orthopedic implants in the treatment of bone deficiencies. Supervisor: G. Bertolino, Co-supervisor: A. Yawny.
- 2014 – 2015: Integrative Engineering Project by Matías Korten, Mechanical Engineering, Balseiro Institute. Title: Development of orthopedic devices based on the Superelastic Effect for the treatment of different bone deficiencies. Link. Supervisor: A. Yawny, Co-supervisor: M. Sade.
- Master’s theses:
- 2018 – : Master’s Thesis by Santiago Price, Mechanical Engineering, Balseiro Institute. Title: Development of Bioinspired Systems actuated by shape memory materials. Supervisor: H. Soul.
- 2016 – 2018: Master’s Thesis by Gastón Alonso, Mechanical Engineering, Balseiro Institute. Title: Utilization of shape memory materials for the correction of bone deficiencies. Link. Supervisor: G. Bertolino, Co-supervisor: A. Yawny.
- Doctoral theses:
- 2018 – : Doctoral Thesis by Gastón Alonso, Mechanical Engineering, Balseiro Institute. Title: Design, characterization, and optimization of orthopedic devices made of shape memory materials. Supervisor: G. Bertolino, Co-supervisor: A. Yawny.
- Conference presentations with proceedings publication:
- 2019: G. Alonso, G. Bertolino, A. Y
