Heat-triggered shape memory

To date, heat-triggered shape memory polymers have had the greatest share of research and application adaptation. However, trigger mechanisms could also be chemo-responsive, e.g., water, ethanol, and pH change photo-responsive, e.g., UV or IR, including radio waves and/or mechano-responsive, e.g., stretching, impact, etc. 

Figure 3.9 Setup used for isothermal stress-strain testing and constrained shape recovery testing. The fixture provided a 1-D external confinement and the furnace was used to trigger the shape memory effect by heating the specimen above its Tg. The MTS machine was used to record the resuiting recovery stress...

An ideal shape-memory polymer for a given application has a small set of parameters that determine the usefulness, or otherwise, for a given application (1) The fractional recoverable strain the maximum fraction of the imposed strain that can be recovered when triggered. (2) The temperature of maximum recovery rate Tmax for a given rate of heating a measure of the temperature required to trigger... 

The Maitland research group is the leader in developing SMPs for clot-removal devices. In 2002, two studies were reported using thermoset polyurethanes for stroke treatment [45,46]. In the first study, an SMP device was manufactured as a wire that first punctures the clot, is then activated to coil on the distal side of the clot, and is then finally retracted for removal (Fig. 3) [45]. Using an in vitro setup, the prototype devices were shown to activate and hold a porcine blood clot at pressures up to ten times that of the neurovasculature. In a follow up study, laser activation of the shape-memory effect was investigated for the coil clot extraction device along with novel umbrella extraction devices and embolic coils [46]. Laser activation allows the devices to be heated past body temperature to trigger the shape-memory... 

The method of activation is the next major practical challenge after an SMP medical device has been shipped to a clinician, assuming all packaging and storing has been performed correctly. Different triggering mechanisms have been proposed to activate the shape-memory effect including the use of body heat, external heat, lasers. 

The most studied SMPs are thermally activated. This means that the shape recovery is triggered by heating the polymer to a temperature above its switching temperature. Marty researchers have achieved the SME on conventionally thermally-active SMPs. Employing hydrophilic or water solirble ingredients in SMPs can accelerate the moisture/water-active shape recovery process. Chen et al. (2007a) developed a water-active shape memory biodegradable polymer from chitosan cross-linked with epoxy. The chitosan is relatively hydrophilic. 

Shape recovery in a particular SMP may be triggered by a few different stimuli. For instance, heat, water, and ethanol are all possible stimuli for polyurethanes and their composites. Additional advantages of SMPs over their metallic counterparts include high recoverable strain and easily tunable properties (http //www.azom.com/article. aspx ArticleID=6038, 13—15). Furthermore, scientific studies have also revealed that almost all polymers intrinsically have shape memory effect (SME) properties, which are also thermo- and chemo-responsive, i.e., the SME in most polymers can be either thermally activated or chemically triggered. 

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