Medical Device Link.

MANUFACTURING

Enabling innovation

Many innovative ideas fall at the first hurdle because available technology cannot fulfil the desires of the inventor to produce devices in sufficient quantity and at a cost that is suitable for the intended market. Prototypes may be made by laboratory scale methods that satisfy the design brief, but in many cases these methods lack the basic process control and measurement required for repeatable device quality and scale-up for manufacture. It is particularly frustrating to hand craft a prototype that works, only to find that the next prototype, made under what appear to be the same conditions, fails. This article describes a case study in which problems of this type have been encountered. It provides examples of solutions to those problems, which enable a route to regulatory approval and commercial marketing.

Figure 1: Smartpoint moulding.

The “smartpoint”device from DRFP Ltd (Sheffield, UK, www.smart-seal.co.uk) was conceived to improve existing root canal filling treatments. The device is used to fill the cavity created when the root and nerve of a tooth are removed, typically because of infection caused by decay or injury. Traditionally, a thin plug and sealant are inserted after cleaning, but this procedure can leave voids in the canal and result in subsequent infection that requires the procedure to be repeated. The smartpoint device comprises a radiopaque core (Figure 1) with a hydrophilic polymer coating, which expands laterally as it absorbs water from the tooth; this creates a tight seal that prevents reinfection, post treatment. One of the main challenges of manufacturing these devices is the production of the central core, which requires micro scale dimensions, high precision tolerances and a high aspect ratio. Tip diameter is 0.2 mm, it is 40 mm long, has a draft angle of 2 degrees and weighs 0.06 gm. In addition, the hygroscopic polymer materials employed must be heavily filled with 60% by weight filler to create suspensions that meet the coating requirements and the radiopacity specifications.

Design challenges

An injection moulding (or variant) process was identified as the most suitable route for manufacturing the cores because it offers the ability to produce high volumes at low marginal cost. However, the design specification posed a number of challenges:

• Material compounding. A multi-component polymer matrix and micro powder filler material must be accurately blended to provide good consistency and the correct degree of radiopacity; it also requires a viscosity suitable for injection moulding.
• Material conditioning. The use of heavily filled hygroscopic materials requires careful material preparation and handling prior to, and during, the moulding process; batches that contain too much moisture can degrade and produce unacceptable mouldings and overdried materials can cause filling problems and high residual stresses within components.
• Cavity manufacture. To achieve the required feature sizes and tolerances, specialised manufacturing techniques were needed to produce cavity forms for the moulding process. The initial pilot cavity form was generated by machining on a high precision, high speed milling machine (Kern, www.kern.co.uk). The subsequent production tool was designed with two impressions; this mould is still undergoing development to finalise the optimum design and allow fully automated production.
• Moulding technology. The product form requires high pressure, high speed injection rates to ensure that the high aspect ratio cavity is adequately filled. The machine must also be able to accurately dose material in a highly repeatable manner to minimise variation of product properties throughout a production batch.

After approaching a number of injection moulding companies it became clear that the product would be extremely difficult to process using conventional technologies. The Centre for Micro and Nano Moulding, based in the Polymer Interdisciplinary Research Centre at the University of Bradford, Bradford, UK, was contacted for advice. Following a detailed study of the product requirements, a plan was developed for material compounding and manufacture of a simple prototype mould for initial trials and proof of concept studies. Microsystems UK (www.microsystems.uk.com), was selected as the provider of the cavities based on its reputation for providing high precision tooling for injection moulding and micro moulding activities using advanced manufacturing techniques such as micro machining and micro electrodischarge machining.

Material compounding

Figure 2: Compounding extruder.

Material compounding was performed initially using a 18 mm twin screw extruder (APV, www.apv.com) with a feeder for each of the thermoplastic materials and another for the micro scale powder filler (Figure 2). Problems were immediately evident using this system, caused by the inability to feed the powder consistently into the extruder. Bridging and aggregation of the material were the main issues and this was resolved by batch mixing the thermoplastic components and the filler material in a tumble blender before feeding it into the extruder. Although this solution is useful for producing materials for proof of concept trials, it is not ideal for a production process, thus further work was performed in partnership with Smith and Nephew (www.global.smith-nephew.com). This has led to the development of a process that uses a more specialised powder feeder and separate feeders for each component, which has been shown to give good results.

Once the candidate material had been produced, it was important to understand the rheology of the filler/polymer suspension to ensure that the high injection rates required to fill the cavity would not cause detrimental material behaviour. To test this, an adapted injection moulding machine, a 100 tonne Roboshot high speed system, (Fanuc, www.fanucrobotics.co.uk) was used, which performs as a high shear rate capillary rheometer. However, the filler content increases the shear rate, which makes it a suitable candidate for the micro moulding process.

Moulding trials

Figure 3: The microsystem.

Moulding trials were performed using a Microsystem 50 machine (Battenfeld, www.battenfeld-imt.com), which offers accurate injection control at high speeds and pressures (Figure 3). More importantly, it can dose volumes of material with accuracies of a fraction of a cubic milli-metre, which ensures repeatability when manufacturing high aspect ratio components of this size. An integrated robotic handling system also allows rapid development of a fully automated process and the machine has an integrated clean room enclosure for medical applications.

A design of experiments trial was adopted to determine the optimum processing window. This revealed that tight control of the material condition and the processing environment were required to produce acceptable products in a reproducible manner. The material conditioning was shown to be particularly important. If the material is not dried sufficiently, then degradation can occur in the extruder barrel to result in brittle cores that snap easily. However, overdrying can have detrimental effects on the flowability of the material and the cavity simply will not fill. Problems increase if conventional plastics driers are used because of the nonuniformity in drying the material from the top to bottom of the chamber caused by the small batch sizes involved.

The method that produced the best results employed a vacuum oven for a short drying time with careful storage of materials before use. Future trials will be performed using a dedicated climatic chamber for material storage to obtain tighter control of material conditioning.

Once the process conditions were determined and the material was under control, a production batch for clinical trials was produced. Initial tests proved positive, but demands for increased radiopacity within the core component necessitated an increase in the filler level and corresponding iterations of the experimental method described above to once again achieve acceptable products.

Next steps

Since then, more than 150000 products have been manufactured and there is ever increasing demand for the device from the dental community.

It has also won a prestigious Plastics Industry Award (www.plasticsawards.com) for Best Technology application. The judges believed the smartpoint project was a good example of innovation and technical collaboration. They said, “The combination of material formulation, sophisticated tooling and micro moulding shows all the benefits of a collaborative approach. This uses innovative polymers in an application where the materials have never been used before.”

Work is now concentrating on further scale up of the process and a new production tool design is replacing the original cavity forms. This tool offers moulding of two products concurrently (more cavities would be detrimental to the process repeatability and product quality) with tighter tolerances and improved product surface finish. The increased outputs offered by the new technology will allow the company to expand into international markets and ensure even greater commercial success.

Dr Ben Whiteside is Technical Manager, Micro and Nano Moulding Centre, School of Engineering, Design and Technology, University of Bradford, Bradford BD7 1DP, UK, tel. +44 1274 236 266, email: b.r.whiteside@Bradford.ac.uk, www.ukmig.com.

Peter Manser is Industry Associate with the Micro and Nano Moulding Centre, University of Bradford, e-mail: peter@accrutek.co.uk.