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Design Chain – Micro-medical parts, perfectly formed

By Lou Reade
Posted 6 December 2012

The use of micro- and even nano-moulding is becoming more widespread in the medical world, especially in the already-demanding field of medical devices.

Injection moulding techniques can now create tiny parts - or imbue normal-sized parts with intricate surface detail. There is little doubt that the medical industry is the main beneficiary of these new methods.

All types of parts, from hearing aids to micro-fluidic devices, are already being made in this way.

Medical sectors have benefitted from micro-moulding

Researchers in the US have studied how injection mould conditions can help to improve the production quality of micro-moulded parts. Their ultimate aim is to create plastic parts whose surfaces mimic the properties of specific biological tissues, in order to influence the way in which cells behave.

John Rodgers, Burak Bekisli and John Coulter, from Lehigh University, moulded high heat crystal polystyrene (HHCPS) and high impact polystyrene (HIPS) macroscale parts with microscale features - so-called 'micro pillars' which were a maximum height of around 15 microns.

They used a commercially available mould base (with matching steel insert) and a micro-featured silicon wafer - as well as an accurate temperature control system. Ultraviolet (UV) photolithography was used to create micro-channels on the surface of the silicon wafer, which acted as the back half of the injection mould surface.

Parts were moulded at 220, 260 and 300F on a Nissei micro-injection moulding machine. The parts were ejected with pins that pushed them away from the micro-featured mould surface.

Cycle times to create the parts ranged from around two minutes (HIPS at 220F) to nearly 11 minutes (HCCPS at 300F). However, the longer cycle times - and higher temperatures - created better parts, as the miniscule 'mould cavities' were filled more effectively. The maximum aspect ratio (height divided by width) was 9.3.

The researchers found no significant difference in the performance of the two materials, though HIPS has a much lower melt flow index (MFI).

"By achieving uniform surface properties - and controlling pillar aspect ratios - a variety of surface compliances can be created, and used to direct biological cell activity," Rodgers told delegates at this year's Antec conference, which was held alongside the NPE exhibition in the US.

In bioengineering, the microfeatures can help to direct and control the biological activity of cells - because cells have a mechanical sensitivity to their environment, and will behave differently depending on what they detect.

Mesenchymal (adult) stem cells, for example, differentiate into different types of cell depending on the elasticity of their environment - which can be altered depending on the structure of the plastic surface.

How small do you want to go?

"Control of cell fate has many important implications as a means to provide cell therapy in the medical field," he said.

Injection moulding machinery maker Desma, in collaboration with KUZ Leipzig, has developed an improved way to make micro-parts from two-component liquid silicone. It says the main uses of the technique will be in the medical industry.

While the material can be used to make more effective parts, it can be difficult to process. A machine to make these parts needs a metering system for feeding and mixing, an injection moulding unit with plasticising unit, heated mould and handling equipment.

"For micro-injection, the screw diameter limits the precision, and the technique requires a large gate - resulting in a lot of waste and long cycle times," said Desma's Bjoern Dormann, in a presentation at Antec.

Desma has optimised its FormicaPlast machine to allow more accurate production of two-component silicone parts.

Firstly, it uses the plunger principle for injection, to give high precision and smooth material handling. At the same time, a transfer index mechanism is used to mould and inject the material.

"For optical microparts in particular, the melting of the thermoplastic is essential," he said.

In this process, the weight of the sprue is much more than the part itself. But using parallel injection units keeps the weight of the sprue to a minimum.

The main aim in the medical industry is to use minimally invasive surgery to implant the micro part into the body. An example is to make micro channels, which transport fluids in order to relieve pain for patients. Another high value application is micro containers. These are filled with medicine, which is then fed by a micro pump into the body. After emptying, the containers can be refilled.

Silicone lenses are another possible application of two component liquid silicone micro-moulding.

So whether micro moulding is used to make tiny parts, or to add intricate detail to the surface of conventional parts, it seems likely that its role will be increasingly important in the medical sector.      

Check out www.prw.com in the coming weeks for further Design Chain features and click here for the digital version of the new supplement.

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