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Method and Apparatus for Adjustably Inducing Biaxial Strain



  • Understanding the dynamic changes in the microstructure of thin films as a function of biaxial strain
  • Inducing biaxial strain in thin films for failure analysis
  • Improving the reliability and performance of devices such as microactuators, biological microsensors, passivation layers, micro-electronics, data storage, and other film-based devices


  • Enables uniform radial or biaxial stretching of a thin film by greater than 2%, allowing for examination of grain boundary changes
  • Fits within the dimensions of a TEM (transmission electron microscope)
  • Can be used in a dual ring configuration to expand and contract the film repeatedly in a single apparatus, making it possible to fabricate a controlled actuator for uses in biaxial fatigue measurements and bi-directional straining experiments
  • Can be used at a larger scale to apply strain to larger samples


Michael Vestel and Daryl Oshatz have invented a method and apparatus for uniformly stretching and compressing thin film radially by more than 2%, a magnitude of strain not attainable using elastic or thermal expansion of ordinary materials. They used a nickel-titanium shape memory alloy (SMA), to make a ring-shaped sample holder for a transmission electron microscope (TEM). The thin film is secured to the sample holder, which responds to a change in temperature by changing the inner diameter. The amount of strain depends on the quantity of heat, allowing experiments to be conducted at a prescribed rate.

The invention overcomes previous problems in studying thin films, including difficulties in generating forces and strains required in a small space, gripping the film, and prevention of bending force components in the film. Although generating linear strain has been possible, it does not allow for study of the types of stresses seen in actual film-based devices, which are typically biaxial.

Another embodiment of the invention allows for the film to be repeatedly compressed and expanded, thus making it possible to fabricate a controlled actuator for uses in biaxial fatigue measurements and bi-directional straining experiments. Another configuration as a thermally activated prime mover of a reversible radial actuator is also possible.


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