Shape Memory Polymer - A Complete Guide
Shape memory polymers are an exciting area of polymer innovation. In essence, they are materials that can ‘remember’ what shape they were and return to that shape when some stimulus, like heat or light, is applied.
The range of applications of such materials is huge, covering medical, industrial, electronics, textiles and others, and development across all of these areas is still ongoing in laboratories around the world.
What is a shape memory material?
There are several types of shape memory material.
As implied in the name, these are materials who have a “memory” of their shape. Specifically, they are able to return to their original shape after they have been deformed in some way. They can return to their original shape when a particular stimulus is applied. This stimulus may be heat or light, for example.
This ability is known as the shape memory effect (SME) and can occur in alloys known as shape memory alloys (SMAs) and in polymers, in shape memory polymers (SMPs), but is also possible in shape memory hybrids (SMHs).
These all belong to a wider set of “smart materials”, also known as intelligent or responsive materials, that have one or more properties that are affected in a controlled manner by some external stimuli. In the case of shape memory, the property that is affected is their shape, by applying some external stimuli such as temperature, stress, moisture, electric or magnetic field, pH, light or chemical compound.
What is a shape memory polymer?
Shape memory polymers are polymer materials that you can deform and then by applying a stimulus, such as heat or light, will return to its previous shape.
Here is an example.
A shape memory polymer can change from a temporary shape that is achieved by deforming it, to a previous shape which is its permanent shape when it is not being deformed.
There are different stimuli that can be used to achieve this. So far this has been achieved using heat, light, irradiation with infrared light, immersion in water, and application of electric or magnetic fields.
Applications for shape memory polymers
There are an almost infinite amount of potential applications for shape memory polymers. The ability to return to a shape after different stimuli are applied allows a dynamic element to product design that could revolutionise certain product types. There are already a series of applications in use in real-world settings and it is likely the range of applications will continue to grow.
Some exciting possibilities include smart fabrics, self-deployable sun sails in spacecraft, intelligent medical devices and self-disassembling mobile phones.
Clothes and other ‘smart’ textiles
Because shape memory polymers can respond to temperature, light, pH and moisture, there are many very interesting possibilities for these materials in fabrics. These include comfort, aesthetics, wound monitoring, protection against environmental conditions, smart controlled drug release, and more. Some examples include:
- Smart breathable garments that can regulate heat and moisture to the wearer’s body.
- Wrinkle-free, anti-shrinkable and crease retention fabrics
- Automotive seatbelts using SMP fibers (secures fibers) which absorb kinetic energy increasing safety
- Skin-care products with moisturizing, whitening, brightening and anti-ageing effects due to controlled release of nutritious ingredients or drugs
- Wound dressing products that deliver a drug release in response to variations in pH or temperature, allowing wounds to heal quickly
- Deodorant fabrics that release deodorants at specific temperatures
- Wearable electronic devices
Automatic disassembly of electronics
Brunel University studied the use of shape memory materials to produce automatic disassembly of products to make recycling of certain electronic products easier. One area was investigating standard engineering polymers to create actuator snap fasteners for mobile phones. These hold or release product components during the product’s use phase. The idea is at the end of life they can become actuators by applying a trigger stimulus such as heat, meaning product disassembly can occur more easily.
Biomedical applications for shape memory polymers
There are a range of potential uses for shape memory polymers for medical applications. This include less invasive, smart medical implants, tissue scaffolds and medical devices.
For example, SMP-based catheters which soften at body temperature, potentially reducing the risks for soft tissue damage of organ injuries during their surgical delivery. Source.
Other biomedical applications include biodegradable self-expanding and drug eluting stents, implants for treating obesity, self-fitting vascular and coronary grafts, and customised orthopaedic devices.
Industrial applications
SMPs are already in use in industrial applications around the world including:
Robotics: Shape memory foams are used in robotics to provide a soft grip when gripping objects. Foams can be cooled to harden and make a shape adaptive grip.
Buildings: shape memory foams are used to seal window frames
Some other potential applications for SMPs include structural components that repair themselves, such as car parts in which the dents are repaired by applying temperature. They may also prove to be extremely useful in aircraft that would morph during flight, such as wings that change shape.
How do shape memory polymers work?
Shape memory polymers have a special chemical structure that means they can return to an original state from a deformed state. The external stimulus could be heat, light, electricity or magnetism, and usually these generate heat within the polymer as the mechanism to start the process of changing from the deformed state back to the original state.
Polymers can occur in two states, either a crystalline state where it is organised uniformly and becomes a rigid relatively strong structure, or an amorphous state where the polymer subunits are randomly scattered and relatively soft and flexible, moving around fairly easily. The difference with a shape memory polymer is that is has a semi-crystalline structure. This means both states occur at the same time within a specific temperature, usually room temperature.
There are different ways that this effect of having a ‘memory’ is achieved, which have different names like “dual-state mechanism”, “dual-component mechanism”, or “partial transition mechanism”. Find out more.
It is useful to explain some of the mechanics that are at play, by looking at the “glass transition temperature”:
“Glass transition temperature” Tg, is the temperature at which the polymer changes from one state to the other, so from crystalline (rigid) to amorphous (flexible). Shape memory polymers have two of these temperatures. In the initial crystalline state the movements of the polymer segments are frozen. This is the initial state that it will return to. When these become heated, the state changes. When temperature is increased the rotation around the segment bonds becomes less impeded, and it is transformed into an amorphous state overall.
This is because within the material there are hard segments, crystalline domains of soft-segments, and amorphous domains of soft-segments. These three segment types always exist. Though only two of these segments elongate. The amorphous and crystalline domains of the soft-segments are the parts that elongate, whereas the hard-segments do not.
In their deformed state, the chain segments which have been put under external stress to deform them are prevented from recoiling into their original state. This is achieved by ‘reversible netpoints’ which act as the molecular switches. These can be formed by physical interactions or covalent bonds.
When the second glass transition temperature is reached, the polymer changes back to its original state. One way of understanding why this happens is that physical systems want to return to a state of most randomness, and less order. So when we add temperature which is an increase the mobility of the chains, it will want to go towards the state of the most disorder. When we stretch it in the first place, goes from a random alignment of the chains, to a slightly less random, because they’re stretched. So, when we increase the mobility of the constituent parts at this stage, and due to the interactions within the semi-crystalline structure, it will want to return to the less ordered, original state.
https://www.youtube.com/watch?v=vuoorVtYWgk&feature=emb_logo
Shape memory alloys versus shape memory polymers
Metal alloys such as nitinol were shape memory alloys studied prior to shape memory polymers, but polymers have several advantages over the alloys.
They can increase in size a lot more, for example, doubling in size versus around a 5% increase for nitinol. Such size increase means more complex geometries can be designed for a variety of applications.
The SMPs also have a softer feel, with a rubbery consistency, which could mean they are less likely to damage surrounding tissue when used in biomedical devices, although in such applications it is vital that thorough tests are carried out regards safety.
Shape memory polymers also have a much lower cost, a lower density, and are easy to process than shape memory alloys. In addition they can sometimes exhibit superior mechanical properties when compare to shape memory alloys.
What are the different types of shape memory polymer?
There are many different types of shape memory polymer and more are being developed all the time.
Three commonly used engineering polymers that can demonstrate the shape memory effect (SME) include polytetrafluoroethylene (PFTE), polylactide (PLA), and ethylene-vinyl acetate (EVA).
Where can I buy shape memory polymers?
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