Polymersome Magneto-Valves

Researchers from the High Field Magnet Laboratory (HFML) and the Institute of Molecules and Materials (IMM) of the Radboud University Nijmegen have demonstrated that high magnetic fields can be used to reversibly open and close nano-sized polymer capsules. This finding makes it possible to capture and release cargo particles, which is a crucial first step towards applications in magnet-assisted drug delivery. The results have been published on September 24, 2014 in Nature Communications.

The nanocapsules are made by deflating hollow spheres, consisting of a polymer membrane, into a bowl shaped structure (Figure 1, left images Figure 2). Previously it was shown that these nanovesicles can be loaded with nanoparticles using chemical methods, such as osmosis. Now it has been demonstrated that magnetic fields can do the same, but in a reversible and more controllable way.

Magneto-valves
When placed inside a strong magnet, the molecules in the membrane of the vesicle align perpendicularly to the magnetic field direction. This alignment leads to stretching of the membrane, the deformation of the capsules and the opening of their mouth (right images Figure 2). The strength of the magnetic field determines the size of the opening, resulting in a reversible process. The vesicles open in a magnetic field, but return to their original closed state once the magnetic field is removed. It is the first time that such a magneto-valve nanosystem has been realized, permitting to load and unload cargo particles in a controlled way (Figure 1).

Nanorockets
IMM scientists had recently demonstrated how to use the vesicles as nanorockets by loading them with a platinum nanoparticle. In the presence of fuel the nanoparticles create oxygen bubbles, which are ejected from the vesicle opening, propelling it forward. Future research is aimed at combining the nanorocket and magneto-valve functionalities, creating a vesicle with an opening large enough to allow propellant to be ejected, but small enough to contain the cargo. When the rockets have arrived at the right location, the cargo can be released by opening the vesicle even further.

Fine-tuning
To reach this goal the technique needs to be fine-tuned. The opening of the capsule mouth-opening at several field strengths needs to be investigated. New membrane molecules should be developed and tested; molecules that are bio-compatible and that respond to lower magnetic field strengths, paving the way for clinical applications using the magnets of a MRI-scanner.

References
P.G. van Rhee, R.S.M. Rikken, L.K.E.A. Abdelmohsen, J.C. Maan, R.J.M. Nolte, J.C.M. van Hest, P.C.M. Christianen and D.A. Wilson: Polymersome magneto-valves for reversible capture and release of nanoparticles. Nature Communications 5, 5010 (2014).

Press release: http://www.ru.nl/hfml/news/news/news-items/magneetveld-opent-en/

Figure 1. Without a magnetic field the vesicle is closed (1). When the magnetic field is turned on the vesicle opens up (2,) allowing the cargo to enter the vesicle. By making the magnetic field weaker or stronger the vesicle will respectively close (convenient during transport) or open and release its cargo (3 and 4).

Figure 2: Electron microscope images of the nanocapsules. The capsules have a small opening when no magnetic field is present (0 Tesla, left). In a strong magnetic field (20 Tesla, right) the vesicles are deformed by magnetic alignment, resulting in a large opening.