British PhD student 3D prints on inflatable substrate to create artificial muscles
Over the next few years, a lot of interesting 3D printed medical innovations are set to become medical realities. 3D bioprinted blood vessels, tissues, even organs are all on its way. However, one Nottingham PhD student shows us that 3D printing technology can do a lot more for the medical world than just bioprinting. For Fergal Coulter, a lecturer and PhD candidate at the College of Art & Design and Build Environment at the Nottingham Trent University, is using 3D printing and scanning techniques to create artificial muscles.
This project is all about a very special kind of artificial muscles: Dielectric Elastomer Actuators (DEA). While perfect for a variety of medical applications – he refers to examples such as cardiac assist pumps and tools that can help with sphincter paralysis - these DEAs can even be used in 3D printed bio-robotic prosthetics, so these can potentially really make a big difference.
And as he explains that these DEAs are essentially rubber membranes covered with electrodes. ‘DEA are created by printing conductive and stretchable electrodes on either side of a thin rubber membrane (in this case Silicone). The membranes can be stacked to become many layers thick. When a voltage is applied to the electrodes, the opposite charges on either electrode attract to each other, and this results in a squashing of the rubber and therefore an overall elongation,’ Fergal explains. A video of that movement can be seen in below. But as you can imagine, these can take a lot of shapes and forms for a lot of different applications.
So where does 3D printing come into with this process? Well, as part of the project, Fergal has begun 3D scanning and 3D printing on inflated structures (silicone balloons)using multiple layers of hard silicone (Shore A 73 Hardness to be precise) to create gorgeous, but functional structures. As he explains, this makes the DEA much more efficient. ‘A critical aspect of DEA is that they work much more efficiently when the rubber is stretched, and held in tension. It is for this reason that I inflate my balloons (to impart a pre-strain). To hold some of this tension I extrude a hard silicone support frame over the entire surface of the 'balloon',’ Fergal tells us.
And the hexachiral structures visible in the photos are simply geometric designs capable of collapsing and expanding uniformly, a very useful when covering something that deflates and inflates. ‘When the pneumatic strain is removed (i.e. the balloon is deflated) after the support structure has hardened/cured, the entire assembly can collapse in a uniform manner, without buckling or wrinkling. When the compression of the support structure is equal to the tension in the balloon membrane, this is termed a Minimum Energy Structure,’ he adds.
As he explains, most of the design work revolved around scanned surfaces and Rhino3D/Grasshopper software, though he had to work with quite a lot of custom code as well. ‘The toolpath geometries and movement speed GCode are generated in Grasshopper also. Rather than using Repetier or similar host software, I had to write my own, as I've a few Arduino controllers and a RAMPS system to keep co-ordinated while I print,’ he says.
3D printing, meanwhile, initially revolved around a Leapfrog Creatr 3D printer – chosen for its z-axis bed and solidity – which was subsequently expanded and customized to suit the project. ‘I used the Creatr as my base platform to build upon. I replaced the hot-end print-head with a spool valve so i could extrude silicone. I mounted a laser measurement device in line with the valve. I programmed this to act as my 3D scanner. I designed a rotating 4th axis (mandrel),’ Fergal explains. ‘In many ways this makes my machine an additive lathe. The mandrels I make are permeable to air, so when i coat them (by spraying them) in silicone, I can inflate the membranes by forcing compressed air into the core or the mandrel, and this then permeates out at the surface - thus causing inflation.’
Check out this unusual 3D printing process here.
The idea is that this research will be continued in the near future, though Fergal is currently in the process of completing his PhD thesis. ‘[I’m] currently looking for a research institution or university that would be interested in continuing this work. I will be publishing my methods over the next few months, so I hope other researchers and makers can have a go printing inflated structures too. It is fun, but quite a challenge!’ Let’s hope he finds the opportunity to continue his fascinating (and potentially very useful) application of 3D printed technology.
