Medical Plastics: Cutting things down to size
By Lou Reade
Posted 8 February 2013
An ongoing pan-European research project called Polytubes – and which includes several UK research partners – is attempting to develop new ways to make microscopic plastic tubing.
The overall objective of the project is to develop a process – and machinery – to make polymeric microtubes and tubular micro-components for medical and non-medical applications.
“Micro manufacturing bridges the gap between nanotechnology and conventional manufacturing technologies,” says Yi Qin, professor of manufacturing technology and systems at Strathclyde University and the project’s technical manager.
The aim is to create a new market for European SMEs, namely the manufacture of micro-products and micro-manufacturing equipment in order to put the European Union in pole position in the manufacture of micro-tubular products.
There are four specific objectives: implement a design for manufacturing methodology for volume production (from hundreds of microns in overall size, down to 2-3 microns for the inner channels); to transfer laboratory processes and equipment (extrusion, shaping and laser processing) to volume production; to integrate the individual pieces of machinery into a process chain and put it into a manufacturing platform; and, to demonstrate technical and commercial viability of the volume production on a full industrial scale, by making appropriate prototypes.
There are 17 partners from across Europe, including four from the UK: Pascoe Engineering (which will make micro-tooling); Imperial College (micromechanics modelling, and process design for mass production); Strathclyde University (designing functional prototypes, and the system for tube shaping); and Birmingham University (product design for mass production).
“If you could make functional and sophisticated tubular components, you could innovate the design of micro-fluidic devices and change the way you make them,” he says Yi Qin.
The Precision Engineering and Micro-Manufacturing Research Group (PEMMRG) at Strathclyde has been involved in developing a machine for hot-embossing of polymeric microtubes.
Hot embossing involves stamping an intricate pattern into a polymer that has been softened by heating it just above its glass transition temperature.
There is an emerging need for tubular micro-components (with diameters of less than 1mm), for micro-medical devices, micro-fluidic devices and heat-management systems.
However, shaping these microtubes cannot be achieved by simply scaling down a conventional-scale process and equipment: many size factors -- relating to the material, process, tool and machine – must be considered.
Specific micro-shaping technologies, which can convert small tubes and thin sections into the required structures, are needed. At the same time, machines to enable mass-production must also be developed.
As a technique for creating surface micro-structures, hot embossing often uses mould inserts for the forming and shaping of polymeric parts. The researchers say that, compared to other processes, hot embossing has less system complexity, shorter production cycle times and lower processing temperatures.
The process is usually used on flat surfaces such as polymeric sheets and thin films, to produce ‘2.5D’ features.
Dedicated tool design
But hot embossing of micro-tubes involves forming 3D features (both outer and inner features), which requires more dedicated tool design and process control – including considerations on the stiffness of tubular structures.
Controlling the formation of inner features is still a major challenge, and requires more accurate definition of process conditions such as material properties, temperature, holding time, pressure and handling of the tube/component.
So far, a mass-production process has been qualified and a novel, miniature, desktop hot-embossing machine and forming tools have been developed.
The desktop hot embossing machine integrates a linear press, forming dies and tool components with heating and cooling units, precision guides and machine frames, as well as an automated micro-tube handling system.
The machine could be used in other applications, such as surface texturing on polymeric thin-films, for use in optical or microfluidic devices, say the researchers.
Some of the other processes being developed by the project, in addition to hot-embossing, include: laser drilling and trimming; blow forming; and cross-rolling of micro-tubes. The eventual aim is to integrate all processes and machines into a single manufacturing platform for making polymeric, tubular micro-components.
“We’ve identified lots of potential applications,” says Yi Qin.
Microscopic devices have been at the forefront of medical science for some time, but have traditionally been made from metals. Now, with new processing techniques, microscopic plastic parts are finally getting in on the act.
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