The medical device and pharmaceutical market requires the fast, homogenous mixing of materials for many applications, for example during drug development and delivery or when creating medical implants. Traditional contact mixing brings challenges for medical device manufacturers who must avoid contamination, meet stringent quality standards, and ensure patient safety to the highest levels. These challenges can be resolved with a non-contact planetary mixing technology. Here Kevin Brownsill, Head of Technical: Learning and Development at adhesives supplier Intertronics, discusses common issues with industrial mixing in medical device applications, and explains how to address them.
Contact mixing, also known as mechanical agitation, can range from stirring with a stick, glass stirrer, electric stirrer with blades, impellers or paddles, or magnetic stirrer. However, when mixing liquids, pastes, and powders, medical device manufacturers may experience issues with this approach, typically variance between operators, difficulty validating the process, poor dispersion, or a high defect rate. Contact mixing offers a lack of traceability and process control that is undesirable for any medical device assembly process.
Productivity is another challenge MedTech manufacturers may want to address with a new mixing approach: increasing throughput or reducing waste. A manual mixing process may have numerous weighing or decanting steps, as well as requiring mixing by hand. If there is a high filler content or heavy fillers in the mixture, the operator may risk repetitive strain injury (RSI) following long periods of hand mixing.
Improving mixing quality
With contact mixing, quality and repeatability issues can occur when the paddle or stirrer introduces air into the mixture, mixing differs between operators, or contamination occurs between batches. Incomplete mixing can risk serious defects, particularly in adhesives applications where insufficient mixing of a two-part adhesive can result in incomplete cure, and therefore a product that cannot withstand its intended environmental conditions.
Furthermore, it may be difficult to achieve a homogenous mix manually with certain material types. For example, combining materials with differing viscosities, or when adding solids such as conductive powders, catalysts, phosphors, fillers or even nanoparticles, into liquids or pastes.
Another challenge of contact mixing is that rollers, blades or propellors can cause physical damage to the components of a mixture. This can be a particular problem in applications where MedTech manufacturers are mixing delicate materials, such as enzymes or nanostructures.
Planetary centrifugal mixers
One alternative to contact mixing is using a planetary centrifugal mixer. This is a non-contact mixing method that combines revolution and rotation within a set radius to achieve a fast, homogenous mix. Rotation is typically at ~1,000s of RPM, generating mixing forces of about 400 G. Users mix in their own containers (with sizes ranging from 12 ml to 20 litre), to mix, disperse, and degas materials in seconds to minutes.
Planetary centrifugal mixers can be programmed with different ratios of revolution and rotation —“recipes” — for example to add more rotation for the defoaming function or more mixing for a more centrifugal action. Altering the speed and mode can help with mixing difficult materials, some of which may require a 10 or even 20 step program.
The benefits of non-contact mixing
One of the key benefits in medical device assembly is that planetary centrifugal mixers are closed cup. Because mixing takes place in removeable containers, the cleaning process is straightforward, saving time, avoiding cross contamination, and making it easier for the technology to be repurposed for other applications.
Adopting a planetary centrifugal mixer can remove steps from a process compared with manual contact mixing processes, saving time and reducing labour costs. Material can be simultaneously mixed, dispersed, and degassed with minimal operator intervention, freeing up team members for more valuable work elsewhere. The technology can reduce process waste by reducing the risk of contamination, while the removed need for decanting reduces waste left in containers. The ability to repeat the process can reduce variation due to operator skill and improve product formulation.
Planetary centrifugal mixing is effective at controlling shear, and is usually quite benign to the products. Additionally, unlike machines that rely on the insertion of paddles or impellers into the material, no air is introduced; in fact, there is a tendency to remove it.
Process traceability
Planetary centrifugal mixers enable precise and repeatable control of the mixing process; process variables are programmable. Some models offer data logging and PC connectivity, giving operators information on RPM, mode, and which recipe is being used. Mixers with communication functions can provide remote control and traceability functions — the user can start/stop operation and report abnormal stop information.
Manufacturers can collect data from their mixer to see the exact RPM in near real-time and validate that the correct program was used for a particular batch — useful for quality assurance and as a development tool. This level of visibility is useful to inform any technical documentation ahead of regulatory submission.
Selecting a mixer
There are various planetary centrifugal mixers available, some that purely mix, some that mix and degas, and some that degas to a high level under vacuum. Machine selection is normally based on two considerations, batch size and the required level of air removal. While many manufacturers opt for a larger mixer for higher volume applications, others operate multiple smaller machines to avoid a large upfront investment, prevent a single point of failure, and increase flexibility.
Some manufacturers may need to guarantee that all air is removed from a particular mixture. In this case, manufacturers can select a machine that mixes under vacuum, which can remove invisible micro bubbles.
Planetary centrifugal mixing medical device development
One example of the use of planetary centrifugal mixing comes from the Department of Automatic Control and Systems Engineering (ACSE) team at the University of Sheffield, which is developing a diagnostic glove that includes several flexible bioelectronic sensors. The sensors are formed from elastomers mixed with various conductive nano or micro particle compounds, like graphite, platinum, and silver, which are 3D printed onto the glove’s material. The process involved mixing nanoparticles into very viscous filled polydimethylsiloxane (PDMS) mixture to produce a graphite composite.
After trialling a THINKY Mixer, ASCE has removed the need for solvents as a mixing aid, removed a step from the process, improved health and safety, and increased repeatability.
Another interesting example comes from the University of Bern, Switzerland, where a team of researchers developing a microfluidic platform for the formation and maintenance of blood micro-vessels benefitted from planetary centrifugal mixing. One of the key production issues was the complete filling of all the microstructures with uncured PDMS. The team wanted to reliably prevent air bubbles becoming trapped in the PDMS. A THINKY Mixer was used successfully to simultaneously mix and degas the material.
A good supplier can address your challenge by doing laboratory evaluations and demonstrations with your materials. Based on previous experience, it can advise on the best mixer for your process.
Intertronics offers the full range of THINKY planetary centrifugal mixers. For more information, visit our website or call 01865 842842 to speak to a member of our experienced team.
