Materials 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
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,
102
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
Medical Device Developments /
www.nsmedicaldevices.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146
