Shape Memory Materials

  • Shape-memory materials attempt to recover their original fabricated shape when they surpass a transition temperature (a narrow temperature band, not a single point) between a low –temperature phase and a high-temperature phase.
  • Some shape-memory materials can exert tremendous force as they attempt to return to their original shape. Some significant changes in the properties of the material can accompany the transition.
  • Metal shape-memory alloys (SMA) have found significant commercial use and currently are seeing rapid growth in medical applications.
  • SMA actuators offer the highest pressure and greatest energy density of any smart materials but are not well suited to rapid cycling.
  • Two other shape-memory categories, shape-memory polymers (SMPs) and shape-memory ceramics (SMC), have not found much use to date. SMPs offer very little recovery force and have slow reaction times.SMCs can tolerate much higher operating temperatures than can other shape-memory materials, but their recoverable strain is quite small.

Shape-Memory Alloys

  • The useful shape-recovery and superelastic properties of shape-memory alloys stem from a transition between two crystal forms: a malleable martensitic phase below the transformation temperature band and a stiff austenitic phase above Ttr .The transition is rapid and readily reversible.
  • SMAs require relatively little energy and - unlike most alloys - do not require atomic diffusion to make the transition between phases.Most commercial SMAs are nickel-titanium (also called Nitinol), copper-zinc-aluminum,or copper-aluminum-nickel alloys.
  • The copper-based SMAs have lower processing and raw material costs than do NiTi SMAs, but NiTi offers superior performance. The materials are available in many forms including bars, strips, wires, tubing, foils, thin films and even a porous NiTi.
  • Shape recovery occurs when an SMA piece undergoes deformation while in the malleable low-temperature phase and then encounters heat greater than Ttr. Recovery pressures can exceed 400 megapascals (60 000 psi). Recoverable strain is as much as 8% (4%to 5%for the copper alloys) for a single recovery cycle and drops as the number of cycles increases.
  • One can use any heat source; magnetic induction and direct resistance heating (passing current through the SMA) offer electronic control of heating.
  • SMAs with two-way shape recovery - the ability to recover both a high-temperature form and a low-temperature form - exist, but they have low-recoverable strain and the low-temperature recovery force is very small.

Shape-Memory Polymers

  • Shape memory polymers (SMP) are special blends of two or more polymers. SMP varieties include polynorborene -, polyisoprene -, styrene butadiene -, and polyurethane-based materials and vinyl acetate - and polyester-based compounds.
  • When an SMP encounters a temperature above the lowest melting point of the individual polymers, the blend makes a transition to a rubbery state. The elastic modulus can change more than two orders of magnitude across the transition.One can easily form an SMP into a desired shape by heating it above the Ttr , fixing the SMP into the new shape,and cooling the material below Ttr.
  • The most suitable applications appear to be custom-fit devices, toys,and novelty items. For example, one could form a spoon handle for the disabled, the handle of a surgical tool, a cast or splint for orthopedic applications, helmet linings, shoe linings or a variety of sporting equipment to improve the fit for a particular user.
  • SMP developers hope to encourage development of medical applications and have put SMP samples through standard biocompatibility tests.