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Scaling up and standardization is where many scientists and users new to magnetic separation processes run into difficulties. Failing to define the conditions the separation was carried out under makes it impossible to standardize your protocol or to scale up batch size successfully.

This can lead to a misunderstanding of the capabilities and reproducibility of magnetic beads separation for larger-scale and manufacturing purposes. To resolve these issues, it is essential to first understand the nature of the magnetic rack itself, and then use this knowledge to create the conditions where standardization and scaling up are straightforward and cost-effective.

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What is Magnetic Strength?

In biomagnetic separation processes, the speed that the magnetic beads travel towards the magnet is the result of multiple variables. In short, it is the strength of the magnetic force acting against the drag force on the beads from the buffer’s viscosity, often described as magnetic strength, or the strength of the magnetic field.

To control magnetic strength, users often focus on factors such as separation time, or attempt to limit magnetic bead losses and avoid irreversible aggregation. This is often reflected in magnetic separation protocol descriptions. However, these factors don’t describe the magnetic separation process and, in fact, are only products of that process.

In fact, neither the strength of the magnet or magnetic field are the correct magnetic parameter to control for, and this can lead to issues with reproducibility when scaling up to larger volumes. It is the magnetic force, not the strength or magnetic field, that is the key parameter that must be specified to determine the strength with which the beads are pulled out of solution at the end of the process.

In addition to specifying the magnetic force, the magnetic field is a distinct parameter that scientists should be aware of. Specifically, the magnetic field applied must have a gradient, otherwise no beads will move. This is commonly overlooked by biochemists new to magnetic separators.

Armed with this essential understanding of the magnetic parameters involved in magnetic separation protocols, the next step is to use these tools to correctly specify your process.

Specifying Magnetic Force 

The most common mistake when scaling up production using classical magnetic separation devices is to use the same magnetic force that was used at smaller volumes. While users may assume that a larger, stronger, magnet equals a stronger magnetic force, this is not true. A stronger magnet does not necessarily provide a stronger magnetic force, even if it generates a stronger magnetic field near it. As such, simply using a larger or more powerful magnet will lead to problems with aggregation and bead loss.

Using traditional open magnetic separators (or simple magnets) the magnetic force changes with the distance. This means that the magnetic force over the beads farthest from the magnet decreases rapidly with the distance across the sample volume. At the same time, the magnetic force over the beads nearest the magnet is extremely high. In these circumstances, the magnetic force strength can’t be standardized because it is not uniform across the magnetic beads, nor well defined.

Magnet ‘strength’ is the intensity of the magnetic force used to separate the beads. The magnetic strength of your magnetic source should be sufficient to effectively pull the beads out of the solution, and safely retain them when the supernatant is extracted. Using a closed magnetic separator rack, or permanent magnet that applies a constant magnetic force, allows you to define the magnetic strength applied to the beads across the entire sample volume.

Standardization

Once the magnetic force is correctly specified, it is possible to standardize your protocol. Magnetic separation protocols should be standardized and well-documented to ensure reproducibility and ease of use. For standardizing the magnetic force, you need to have a constant magnetic force system. By using a constant magnetic force, users can establish a maximum magnetic force to avoid aggregation. Similarly, specifying a minimum magnetic force will maintain the correct separation speed and avoid bead losses.

In conclusion, we addressed the challenges in scaling up and standardizing magnetic beads separation protocols. Often, the lack of defined separation conditions hinders successful protocol standardization and scalability. Understanding the nature of the magnetic rack is crucial for creating conditions conducive to cost-effective and straightforward scaling. The present chapter delves into the misconception surrounding magnetic strength and advocates for focusing on magnetic force, not strength or field, as the key parameter for reproducible scaling. It emphasizes the importance of a constant magnetic force system for standardization, allowing users to define, control, and document the magnetic force, ensuring reproducibility and efficiency even at larger volumes

 

Magnetic force profile for a classical magnetic separator (gray) and a smart Magnetic Bead separator (orange). A standardized magnetic bead separation process should define a range of magnetic force that avoids both losses of the beads and the generation of clumps.

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Lluis M. Martínez | SEPMAG Chief Scientific Officer

Founder of SEPMAG, Lluis holds a PhD in Magnetic Materials by the UAB. He has conducted research at German and Spanish academic institutions. Having worked in companies in Ireland, USA and Spain, he has more than 20 years of experience applying magnetic materials and sensors to industrial products and processes. He has filed several international patents on the field and co-authored more than 20 scientific papers, most of them on the subject of magnetic particle movement.

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