At a Glance
Researchers at Colorado State University have developed a device that, upon heating, deploys anchors in all directions. This outwards force is used to permanently lock the device into a larger cavity or cylinder (e.g. PVC pipe or intramedullary cavity of a long bone) to selectively control placement of the device. The mechanism for the expansion of these shape memory anchors is an internal stress associated with a crystallographic transition.
Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape induced by an external stimulus (trigger), such as temperature change. One of the first conceived industrial applications was shape memory (SM) foam in robotics to provide initial soft pretension in gripping. Since then, these materials have seen widespread usage in construction (e.g. foam which expands with warmth to seal window frames) and sports wear. Polyurethane SMPs are also utilized as an autochoke element in engines. And although the potential to develop SMPs for greater applications remains, many have yet to be realized. Specifically, shape memory devices are not currently used to anchor inserts into a sleeve, as implants into a bone, or to control pivoting.
Described is a cylindrical device that, upon heating, deploys anchors outward in all directions. This outwards force can be used to lock the device into a larger cylinder such as a PVC pipe or intramedullary cavity of a long bone, or to selectively control a spring-loaded or auxetic device. The mechanism for the expansion of the anchors is an internal stress associated with a crystallographic transition from martensite to austenite phases.
In order for this to be accomplished, the anchors, at minimum, are made of nitinol, which is approximately equiatomic nickel and titanium.
The general construction of the device is as follows: a hollow cylinder shape is formed, and slats are cut, as can be seen in Figure 1a, below. Next, a fixture is built that holds the anchors out to their desired set points (the position they should be in when expanded) as can be seen in Figure 1b, below. Then a heat treatment is applied in order to set this as the austenitic shape of the material, in other word the shape of the device when above the austenite finishing temperature (Af). Then, the device is cooled below Af which allows it to be deformed such that the anchors are bent back inwards, and a cylinder shape is again achieved. In this state, the device can be inserted into a hole or a larger cylinder.
Once heat is applied and the temperature of the device surpasses Af, the device will try to switch back into its “remembered” austenite shape with the anchors deployed. This internal stress that deploys the anchors can then be harnessed to lock the device into a rigid area or expand to a flexible area surrounding the device. Figure 1 shows a schematic of the anchor deployment, and the image above shows a prototype device doing the same.
- Temporary and modular anchoring
- Can be used to control pivoting an even stiffer anchor in place
- Medical devices (e.g. orthodontics and orthopedics)
- Control pivoting
A. Morrone, et al. (2020) A Novel Design Methodology for Osseointegrated Implants using Shape Memory Alloy Anchors. IAE. Biomed Sci Instrum Vol 56(2) P 350-363
A. Morrone. A Novel Design Methodology for Osseointegrated Implants and the Effects of Heat-Treatment on Shape Setting Nitinol Foil. Colorado State University - ProQuest Dissertations Publishing, 2020. 27829767.
Last updated: October 2022