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magnetic bead coatings

Conclusions on the magnetic bead coatings post series

The goal of the series of posts from the last weeks was to review the state-of-the-art of magnetic beads coatings. The contributors have reviewed the classical surfaces, but also the new approaches to improve and simplify the process. Last but not least, the physical aspects of the magnetic beads and the separation process were discussed, as they have a critical impact on the success of the coating process.

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Dr. Fabrice Sultan (Merk Chimie) has reviewed the most popular surfaces: plain for passive adsorption coatings, modified surfaces (carboxyl, hydroxyl, amino) and pre-activated (tosyl, epoxy, and chloromethyl) for covalent binding and bio-activated (protein A, protein G, streptavidin, biotin) for non-covalent binding. Today, the most common surfaces are still the covalent bindings, but as the use of magnetic beads is expanding, it is necessary to work with surfaces that allow rapid development of the coatings and control of the orientation and coating-efficiency of the capture molecule.

This is the reason why Tosyl-activated magnetic beads are becoming a popular alternative. Dr. Michael Jansen (NBCL) describes in chapter 3 the handling and advantages of the use of Tosyl activated magnetic beads in chemiluminescent immunoassays. As he explains with detail, this surface is ready to use and does not require pre-activation. In addition, Tosyl activated beads show an excellent lot-to-lot reproducibility.

Looking for tomorrow’ solutions, we asked experts working at companies that are introducing new products for surface coating with capture biomolecules. Josh Soldo (Anteo Dx) focused his article on the limitations of classical surfaces to meet the requirements of ultrasensitive assays, paying special attention to the density or parking area of the ligand and to the ligand conformation and its orientation. He proposes new approaches, such as the titration of the amount of ligand per surface area unit (instead of using molar excess) or the metal polymer chemistry. He concludes that these new approaches will be the key for the development of the next generation of immunoassays. 

Another innovative approach has been presented in the article by Prof. Stephen Henry (Auckland University of Technology and KODE Biotech). He described novel water-dispersible self-assembling molecules, called function-spacer-lipids, which are able to coat virtually any surface with almost any biological or non-biological material. Using this technology, a magnetic bead can be modified with an appropriate FSL construct and used to specifically capture live cells, virions, particles, or other biological or non-biological material

However, the success of the coating process not only depends on the selected surface and activation protocols. The last two articles of the series pay attention to two factors often overlooked when developing a new test. Dr. Sergio Rubio (Ikerlat Polymers) reviewed the importance of the physical properties of the magnetic beads. The size of the bead will influence the specific area and the separation time, but also the distribution. The effect of the magnetic charge is also analyzed, as higher charge means fastest separation but also higher density. The trade-offs for each development would require the selection, not only of the surface but also of the physical characteristics of the magnetic beads. Dr. Rubio emphasizes the impossibility of having a unique reference of magnetic dispersion that will work on every process, but the need of adjust the physical properties for any single process.

The last article analyzed the two main problems that the magnetic separation rack can generate when used for washing the magnetic beads during the coating process. Even if the biomagnetic separation is the fastest, cheapest and easiest to use technique to separate the solid phase, Dr. Lluis M. Martinez (SEPMAG) points that not paying attention to two critical points may jeopardize all the coating protocols. The use of the wrong magnetic separation rack can imply important losses of materials and irreversible aggregation problems. These problems are not always easy to be detected at small volume and are the main causes of trouble when the protocols are scaled up. Reviewing the physics behind the separation problems, he encourages the use of biomagnetic separation systems using homogenous force.

As editors of this series, we want to acknowledge the effort of all the contributors, specially their quick and enthusiastic response after contacting them to explain this project. Their openness to share their knowledge with the magnetic bead’s user community is an encouraging sign of the potential of this technology for helping IVD and biotechnology communities and by extension humankind.

Related articles:

Bio Sergi Gassó

Dr. Lluís Martínez

FREE Download: Basic guide to magnetic bead cell separation

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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