Applicability of shape memory alloy wire for an active, soft orthotic

Leia Stirling, Chih Han Yu, Jason Miller, Elliot Hawkes, Robert Wood, Eugene Goldfield, Radhika Nagpal

Research output: Contribution to journalArticlepeer-review

60 Scopus citations


Current treatments for gait pathologies associated with neuromuscular disorders may employ a passive, rigid brace. While these provide certain benefits, they can also cause muscle atrophy. In this study, we examined NiTi shape memory alloy (SMA) wires that were annealed into springs to develop an active, soft orthotic (ASO) for the knee. Actively controlled SMA springs may provide variable assistances depending on factors such as when, during the gait cycle, the springs are activated; ongoing muscle activity level; and needs of the wearer. Unlike a passive brace, an active orthotic may provide individualized control, assisting the muscles so that they may be used more appropriately, and possibly leading to a re-education of the neuro-motor system and eventual independence from the orthotic system. A prototype was tested on a suspended, robotic leg to simulate the swing phase of a typical gait. The total deflection generated by the orthotic depended on the knee angle and the total number of actuators triggered, with a max deflection of 35° While SMAwires have a high energy density, they require a significant amount of power. Furthermore, the loaded SMA spring response times were much longer than the natural frequency of an average gait for the power conditions tested. While the SMA wires are not appropriate for correction of gait pathologies as currently implemented, the ability to have a soft, actuated material could be appropriate for slower timescale applications.

Original languageEnglish (US)
Pages (from-to)658-662
Number of pages5
JournalJournal of Materials Engineering and Performance
Issue number4-5
StatePublished - Jul 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General Materials Science
  • Mechanics of Materials
  • Mechanical Engineering


  • Gait
  • Orthotic
  • Shape memory alloy springs


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