Traditional contact mixing can be challenging for medical device manufacturers who must avoid contamination, meet stringent quality standards and ensure the highest levels of patient safety. These hurdles can be overcome 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 rod, electric stirrer with blades, impellers/paddles or a magnetic bar (or flea).
However, when mixing liquids, pastes and powders, medical device manufacturers may experience issues such as variance between operators, difficulty validating the process, poor dispersion or a high defect rate.
Contact mixing is often associated with a lack of traceability and process control, which is undesirable for any medical device assembly process.
Productivity is another challenge that manufacturers may want to address during mixing by increasing throughput or reducing waste. A manual process may involve 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, the process differs between operators or contamination occurs between batches.
Incomplete mixing can risk serious defects, particularly with adhesive applications when insufficient mixing of a two-part adhesive can result in incomplete cure. The result might be a product that cannot withstand its intended environmental conditions.
Furthermore, it may be difficult to achieve a homogenous mix manually with certain material types — such as when combining products with differing viscosities or adding solids such as conductive powders, catalysts, phosphors, fillers or even nanoparticles into liquids or pastes.
Another issue with 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 when 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 in the range of thousands of RPM, generating mixing forces of approximately 400 G. Users mix in their own containers (with sizes ranging from 12 mL to 20 L) 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 a 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 programme.
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, again 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; plus, eliminating the need for decanting reduces the amount of waste left in containers.
The ability to repeat the process can reduce variation owing to operator inconsistency 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
With programmable variables, planetary centrifugal mixers enable the precise and repeatable control of the mixing process.
Some models offer data logging and PC connectivity, giving operators information about RPM, mode and which recipe is being used. Mixers with communication functions can provide remote control and traceability functions; as well, 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 programme was used for a particular batch, which is useful for both quality assurance and as a development tool. This level of visibility helps with 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.
Although 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 companies may need to guarantee that all air is removed from a particular mixture. In this case, they can select a machine that mixes under vacuum, which can remove invisible microbubbles.
Planetary centrifugal mixing during 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 microparticle compounds, such as 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.1 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 microvessels benefitted from planetary centrifugal mixing.2
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.
References
- www.intertronics.co.uk/2022/08/improving-mixing-accuracy-and-repeatability-for-3d-printed-bioelectronics-research/.
- www.intertronics.co.uk/2018/01/case-study-mixing-and-degassing-pdms-for-microfluidic-manufacture/.