After being severely and quasi-plastically distorted, shape memory materials (SMMs) are able to recover their original shape when a right stimulus is applied1. This phenomenon is known as the shape memory effect (SME), and it has been found in a number of material systems, including some alloys, polymers and ceramics etc2-6. Technically speaking, the procedure to fix the temporary shape (distorted shape) is known as programming within the SMM community, and it is followed by the shape recovery process to complete a full cycle.
Traditionally, we call those polymers with SME properties: shape memory polymers (SMPs). Continuous efforts have been devoted into modifying existing SMPs and developing new ones for tailored properties and special functions7-9. Almost all existing SMPs fall into three major categories according to the types of stimulus applied to induce the SME, namely thermo-responsive SMP (normally by means of heating, including inductive heating, joule heating, mechanical heating and light heating etc), photo-responsive SMP (light with different wavelengths but without any heat involved), chemo-responsive SMP (chemicals, such as water, ethanol etc) 10-12.
In fact, 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 (PUs) and their composites. Additional advantages of SMPs over their metallic counter-parts include high recoverable strain and easily tunable properties etc13-15. Furthermore, recent scientific study also reveals that almost all polymers intrinsically have SME properties, that are also thermo-and chemo-responsive, i.e., the SME in most polymers can either be thermally activated or chemically triggered.
There are three major possible working mechanisms for the SME in polymers. Take a heating responsive polymer as an example:
- We can heat it to above its glass transition temperature (Tg) and deform it easily (since it is in the rubbery state). After cooling back to below its Tg, followed by unloading to remove the constraint, the distorted shape is largely maintained, apart from some elastic recovery (since the polymer is in the glassy state). Heating again to above its Tg, micro-Brownian motion enables the polymer to return to the original shape. Poly(methyl methacrylate) (PMMA) is a typical example of a polymer this mechanism applies to (Figure 1.).
- In the second working mechanism, there are two segments/domains in a polymer, in which one is always elastic (elastic component), while the other is able to significantly alter its stiffness, depending on whether the heat is applied (transition component). PU, which has a typical hard/soft-segment structure, is a good example for this type of mechanism (See Figure 2.).
- The third mechanism utilizes the polymer itself as both the elastic and transition component. When wax is heated to achieve partial melting, the solid part acts as the elastic component, while the melted part functions essentially as the transition component. The SME in wax is demonstrated in Figure 3. In order to be more convincing, after indentation at 70oC, which is in the middle of its melting range, and then cooling back to room temperature, the top surface of wax sample is polished to eliminate any hint of indent. Upon heating to 70oC again, protrusion is formed, which is due to the SME properties of the wax.
Practically, we may combine these mechanisms together to achieve tailored performance, such as optimized shape recovery temperature and higher recovery stress etc16,17.
Figure 1. SME in PMMA.
Figure 2. SME in PU.
Figure 3. SME in wax.
Since we can almost always find a suitable chemical to soften a polymer or the transition component (by means of, for instance, plasticizing effect), most polymers are intrinsically chemo-responsive as well. Figure 4. shows a piece of hydrogel. After being compressed, it is able to recover its original shape either by heating or by absorption of moisture (even without any apparent swelling). On the other hand, in order to have the real photo-responsive SME without light induced heating at all, a polymer needs to be specially designed to have the transition component which is able to alter its stiffness in responding to light18. Hence, the photo-responsive SME is limited to certain polymers only.
Figure 4. SME in hydrogel. Left: after compression; right: after shape recovery.
One of the most interesting recent developments is the multiple-SME, in which a polymer is able to recover its original shape through one or more intermediate shapes19,20. It has been proved that the multiple-SME or at least triple-SME (i.e., there is only one intermediate shape between the temporary shape and original shape) can always be realized by programming21. Figure 5. reveals the programming procedure and triple-SME in a piece of PU.
Figure 5. Multiple-SME in PU. (a-c) Programming; (d-f) shape recovery.
In some polymers, there are more than two transitions upon heating, e.g., one is the glass transition and the other is melting. Upon heating to just above Tg, shape recovery is only partial. However, upon further heating to around the melting temperature, full recovery can be achieved. Refer to Figure 6. for an example case of PTFE.
Figure 6. SME in PTFE. (a) Original shape; (b) after stretching at room temperature; (c) after heating to 100oC; (d) after heating to 300oC.
Conclusions and Future Applications
The potential applications of SME enables us to reshape our design in many ways. From biomedical devices to space missions, from smart textile design to active disassembly of obsolete electrical devices, SME is gradually changing our fundamental design methodology 9,10, 22-27.
For example, the potential motion generation characteristics of SMPs are highlighted by; Figure 7. demonstrating the concept of using retractable ethylene vinyl acetate (EVA) stent. While Figure 8. reveals the self-tightening function in a commercial Polylactide (PLA) based biodegradable surgical staple for the medical industry.
Figure 7. EVA retractable stent. (a) Original shape; (b) after programming; (c) after shape recovery.
Figure 8. Biodegradable PLA surgical staple with self-tightening function upon heating.
In addition to motion generation, SME can also be used as a new fabrication technology. Figure 9, shows how different types of surface patterns at different scales can be produced on the surface of polymers by utilizing their SME properties.
Therefore, SME properties in SMPs could offer scientists and industry a world of opportunities.
Figure 9. Surface patterning atop polymers (utilizing the SME).
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