Feature Article
Published: February 1, 2010
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Micro Moulding and Nano-Structuring
The technologies, equipment and skills required to manufacture micro-components or for micro or nano-structuring of devices are reviewed here. Application examples illustrate the latest possibilities when working at these dimensions.
Going to the nano-level
The continuing industrial interest in micro-components and micro- or nano-structuring of devices is being driven by increasing needs for point-of-care diagnosis, portability, minimally invasive surgery, and other factors outside the medical device industry that require miniaturisation. In addition to the established know-how in production micro-moulding, there is a technology push from research and development as methods are developed to design at the nano-level and to achieve specific component functionalities.Moulding processes
Micro-moulding has now been established as a specialised injection moulding process for several years. Whereas conventional plastic injection moulding machines are adequate for producing many small plastic medical parts, a dedicated micro-moulding machine offers a degree of control and repeatability for the moulding of small components down to less than 0.005 g part weight that cannot be achieved with a conventional machine. Current commercially available micro-injection moulding machines inject the material into the mould using a plunger of 3-5 mm diameter, and are designed specifically to make miniature parts as small as 0.005 g in weight. Advantages of using a dedicated micro-moulding machine include:
- Much smaller feed and gating system resulting in less pressure drop to the mould cavities and less wastage of expensive medical-grade moulding materials.
- Injection of small quantities of material with precise position and speed control via the miniature injection plunger, which is driven by a servo electric drive.
Other advantages can include an integrated clean room cell, integrated handling and camera inspection and packaging system in a small envelope, and higher dosage repeatability as a result of the metering and injection system design.
Mould design and manufacture
Design of micro-moulds requires attention to be given to factors such as precise alignment of mould components, support of the mould under high injection pressure, gate size and degating, venting of the mould, a reliable ejection system and part handling on ejection. If any of these factors are not correctly designed, and the parts are not made with the correct tolerances and materials, the process will fail. Mould alignment may be necessary to less than 5 µm, and gate diameters are typically from 60 to 200 µm. Venting is typically assisted by vacuum to avoid air traps when the part is filled. Filling of the runner system and cavities can often take place in less than 0.1s.
The processes used to make micro-moulds are a combination of refined traditional toolmaking processes and more specialised processes. Traditional toolmaking processes would include high-speed milling, electro-discharge machining and wire-erosion. The equipment for these processes can now be highly controlled and precise especially when used in a temperature-controlled environment. More specialised processes would include diamond cutting and a range of nano-manufacturing processes described below.
Nano-structured inserts and replication in plastic
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| Figure 1: Part for a medical staple. The pitch of he teeth is 0.3 mm. |
The medical field’s interest in submicrometer structures on surfaces is, for example, in areas where components are in contact with fluids or where optical functions are needed. Structured surfaces of this type also have biotechnology applications when used in contact with cells, and in other fields such as anticounterfeiting and unique identification.
Table I summarises different nano-manufacturing processes and their typical capabilities in terms of mechanical feature size. These processes can be used to structure metallic surfaces that can be used directly or can be replicated in plastic by a moulding method.
The features on the surface of a compact disc or digital video disc are submicrometer in size and have been made for many years by injection moulding. The master pattern is created by laser structuring, this is replicated by nickel electroforming, and the nickel “shim” is used as the face of a mould insert. Features in the order of 200 nm in size can be replicated effectively via injection moulding.1
The equipment used for the processes in Table I is highly specialised and mostly exists only in research establishments or other specialised laboratories. To utilise it, companies need to develop working relationships with these types of establishments and develop ways to make use of surfaces made by these methods on engineering components. For example, to take a nickel electroformed shell from an electron-beam machined silicon master, machine this to size, and fit it into an injection mould tool to mould plastic parts requires engineering development and expertise. Nickel has a limited lifetime in a mould tool compared with hardened tool steel. For this reason, hardened steel is always preferred by moulders and recent efforts in some centres have developed methods to etch optically diffractive elements into steel.
Micro-machining and micro-moulding capabilities enable the manufacture of micro-devices for many applications, examples are discussed below.
Devices for treatment and surgery
Applications of micro-machining and micro-moulding include certain types of cancer treatment, small moulded catheter tips, micro-needles and small surgical instruments. Another application is in dental surgery, where a new type of filling and infection control has been developed for root canal treatment using a micro-moulded X-ray opaque material with a hygroscopic coating.2
Small slots or radii in metal parts or mould inserts can be cut to <0.050 mm in width, and plastic parts can be micro-injection moulded in mould tools using steel ejector pins <0.20 mm in diameter for part ejection. Holes <0.1 mm in diameter can be moulded, and when holes too small for moulding are required, hole sizes of <0.030-mm diameter can be produced by laser machining.
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| Figure 2: Diffractive optical structure produced in hardened steel and used for beam shaping. |
Figure 1 shows a development part for a medical staple. This is micro-moulded in a four-cavity mould with a cycle time of approximately 6 s. The pitch of the teeth is 0.3 mm.
Fluids and optics
Design and structuring of specific surface features at the nano-level (below 1 µm in size) can modify the wettability of a surface to help control the movement of fluid, a phenomenon widely seen when studying nature. This type of effect requires a combination of the correct surface chemistry and mechanical micro- or nano-structure.
The structuring processes highlighted in Table I can also be used to manufacture diffractive optics, for example, to shape the beam of light emitted from a laser or light emitting diode needed for analysis work. The use of diffractive optics allows flatter, thinner lenses to be used and can achieve results not possible using refractive optical systems. Figure 2 shows a diffractive optical structure produced in hardened steel and used for beam shaping. This was produced by a combination of processes and employed in plastic moulding.
A relatively new application is the interaction of cells with synthetic nano-structured surfaces, which is the subject of current research. An example is cell contact guidance to differentiate stem cells.3
Implanted devices
| Table I: The different nano-manufacturing processes and their typical capabilites by size of feature. | |
| Process | Capability |
| Diamond cutting | Features >5 µm size with Ra range 3–30 nm |
| Direct laser machining | Holes and line features >10 µm diameter/width |
| Laser interference lithography | Lines and interference patterns >1 µm in size |
| LIGA | High aspect ratio features >5 µm footprint |
Some treatments require bioresorbable devices to be used or structural components to be permanently implanted. The scale of parts that is possible with micro-moulding, that is, parts <1 mm in size with features much smaller than this, opens up new possibilities. At this scale, the problem can become one of handling rather than manufacturing. However, micro-moulding also opens up opportunities for parts of slightly larger scale with micro- or nano-surface features or holes. These could be types of clips or staples, devices for drug delivery, bone-replacement implants or possibly components used in neurosurgery.
Design and integration skills
Other applications are starting to be found. The key to manufacturing success in these areas is the know-how to design at the micro- and nano-scale and to integrate the necessary cutting-edge technologies to bring a final product to production. In parallel with this, there is an ongoing need for designers of devices to be educated with respect to the capabilities of these processes.
References
K. Mönkkönen et al., “Replication of Sub-Micron Features Using Amorphous Thermoplastics, Polymer Engineering & Science, 42, 7, 1600–1608 (2002).
2. B.R. Whiteside and .P Manser, Micro Moulding: The Route to a Successful Product, Medical Device Technology, 20, 2, 18-21 (2009).
3. I.H. Jaafar et al., “Micro and Nanomolded Surface Structures for the Proactive Stimulation of Human Mensenchymal Stem Cell Differentiation, Polymer Process Engineering 09, University of Bradford, UK, ISBN 13 978 1 85143 262 2.
Paul Glendenning is Business Development Manager at Micro-Components & Polymer Optics, Micro Systems (UK) Ltd
101 Golborne Enterprise Park, Warrington WA3 3GR, UK
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