Shape-shifting plastics

Shape-memory polymers (SMPs) are the very definition of smart materials. Able to modify their physical characteristics – shape, stiffness and size to name a few – with the application of a huge range of external stimuli, they open up a host of possible applications across many industries, and the medical devices sector is no exception.

At the molecular level, SMPs have a matrix of polymer that resembles a fishing net. This matrix can be deformed – stretched or bent into a different shape – but unlike a normal polymer, which will return to its original configuration when the tension is removed, an SMP can be fixed at its stretch point by applying a stimulus, often heat, only returning to its original shape when another stimulus releases the tension to unlock it. Imagine an elastic band that is stretched to its full extent and stays there until its temperature changes or an electrical charge is applied.

“In the fishing net structure, the points at which the strands interlock are crystalline or hard in some way compared to what connects them, not dissimilar to spider silk,” says John Hardy, senior lecturer in materials chemistry at Lancaster University. “The toughness comes from the ability to stretch and unfold the individual sheets in the matrix, and peeling back the individual sheets means they absorb a high level of energy.”

With SMPs, there are soft and hard segments and reversible transformations are possible because the soft elements that link the hard primary structure elements can store a temporary shape above a certain temperature. For example, some SMPs remain stiffer and locked in place when cooler, but then melt on heating so they can relax into their original form. The ability to engineer a myriad of different shapes and a vast range of triggers – heat, electric charge, magnetic field, light and water to name a few – offers huge potential for use within the human body.

A tuneable advantage

In medical devices, the reason SMPs are emerging as an important class of materials is that they could simplify some medical procedures, support minimally invasive techniques or give rise to entirely new treatment modalities. “We are starting to see applications that derive from many externally applied stimuli, not only light and heat, and that can perform functions in the surgical environment,” Hardy explains.

A combination of low toxicity and high tuneability, plus the potential for biodegradation and resorption are what make SMPs suitable for use inside the body. As well as being responsive to thermal triggers, they can change form because they contain hydrolytically or enzymatically sensitive bonds, meaning they respond to water molecules and the activity of enzymes respectively. Biodegradable SMPs are in use or under examination for use in areas such as embolisation, tissue engineering, wound closure, drug delivery and stent implants. “The low-lying fruit is sutures, which can be sewn in and then swell to fix in position. Later, you can apply the stimulus to remove them or break them down,” says Hardy. “Now, we are starting to explore splints and stents and even tools to guide devices into an area within the body using, for instance, a hook made with an SMP or shape memory metal,” he adds. “An example could be a device for inserting a catheter using microsurgery through an incision. The hook could be guided by SMP properties using triggers in the catheter, such as feeding in light or electricity through an optic fibre or wire.”

Already, there is a long list of medical SMP applications under investigation. That list includes bond defect fillers, aneurysm occlusion devices, cardiac valve repairs, clot removal devices, endoscopic surgery sutures, vascular stents, kidney dialysis needles, pharyngeal mucosa reconstruction, and even tools to conduct surgery inside living cells.

The next step in sophistication

The success of these applications is dependent on the use of stimuli that trigger the SMP to take its original form. Most common are physical triggers, such as temperature, UV light, electricity or magnetic fields. But there are also chemical triggers – water, solvents, biological agents and changes in pH level. In their latest iteration, some SMPs are responsive to multiple stimuli.

These ‘intelligent SMPs’ can perform different functions depending on the trigger and are created by complexing polymers and composite materials with different properties like degradability, electric conductivity, magnetic conductivity, energy storage and antibacterial properties. Each stimulus has its own advantages. For instance, using electricity means the current is tuneable, so applying a different voltage results in a different degree of deformation, which offers more precise control. At Lancaster University, Hardy’s lab is taking the sophistication of SMP applications to the next level using an upgraded nanoscribe – a 3D printer using near-infrared light – to print electronic circuits inside the polymers.

“We are taking transparent SMP matrices and swelling them with monomers, then essentially printing an electronic circuit inside a polymeric matrix,” he says. “So, we can print something with contact pads on the surface, and wires through the polymeric matrix, which then protrude on the other side. We can print inside an SMP in its fixed or elastomeric state, print inside, transform it to make it easy to implant and then expand it to assume a shape in a specific space within the body.”

The research is currently focused on bioelectronics and biophotonics, he adds, as there are all sorts of electronic interfaces in the body. This opens up the possibility of devices that can target nerve interfaces for neuromodulation, tissue scaffolds and much more. Neuromodulation, which already happens with traditional cardiac pacemakers, could be taken much further using this method. Electrodes in the brain could control shaking symptoms in Parkinson’s disease, for example, or devices implanted in the spinal cord or the peripheral nervous system could restore functions lost through degenerative diseases.

“In the peripheral nervous system, SMPs with electronic circuits inside could reconnect parts that have been disconnected, or they could be used to switch off pain, or in nerve interfacing,” Hardy explains. “In cases of spinal cord injury, we could reconnect the brain to the nervous system to regain the use of a limb or to regulate toilet functions. You could potentially heal the peripheral nervous system.”

Currently, Hardy is looking at the specific set of signals in the brain that lead to epileptic fits, with a view to creating an implanted device that could respond in real time to suppress them. Other potential uses of SMPs with imprinted circuits include drug delivery systems with electronic control systems. For instance, a sensor system that detects analytes in bodily fluids associated with certain disease states could trigger the delivery of medication to alleviate symptoms.

A broad horizon for innovation

As more and more potential applications for SMPs emerge, there will be increased focus on the development of new materials. In the development of biodegradable SMPs, for example, new polymer classes, developing responsiveness to new stimuli, and thorough characterisation of materials properties and performance in vivo will play key roles in shaping the development of new medical devices. A wide variety of polymers exist, including many that are biodegradable, but what lies ahead is a thorough examination of their shape memory properties. For all polymers, whether biodegradable or not, more investigation on biocompatibility and toxicity will be as essential as investigating the response to different triggers.

“Designing systems that are responsive to multiple stimuli opens up applications that might have been impossible in the past,” says Hardy. He adds that for now, many of the practical examples are seen in other industries, using the building and construction as an example. Here, he says, SMPs have been used to create adapting insulating foam that expands and fixes itself, which could even have implications for the use of foam technology in marine salvage operations to transport material out of sunken vessels like the Titanic.

Part of the reason we haven’t seen developments as fast in the medical device industry has to do with concerns about patient safety. “In principle, when there are no ethical concerns, the market opens up faster,” says Hardy. “One challenge with electronic interfaces in the body is that they are made with metals or alloys, which causes an inflammatory response.

Patient safety concerns are the reason the bar for market entry is set so high by regulators, and along with finding the best materials for SMP applications, Hardy says working on ways to make them more bioavailable will be the key to getting them into the clinic. “We want something to be usable in surgery but soft enough to be accepted by the body, so engineering and medical acceptance are the two prongs of research,” he concludes. 

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 that 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.

Source: British Plastics Federation