Methods for magnetic manipulation on a micro-scale are many and varied, profiting in most cases from the versatility of magnetic nanoparticles. But using these particles in biomedical applications poses something of a challenge, in large part due to the fact that they're often laden with heavy metals. Now, Savas Tasoglu and colleagues have come up with a strategy for manipulating magnetically tunable 'microcomponents', which avoids these potentially poisonous inclusions (Nature Commun. 5, 4702; 2014).

Non-magnetic techniques for massively parallel self-assembly are quick and inexpensive, but many suffer from low precision and insufficient yield, and overproduction can lead to costly redundancies. Tasoglu et al. overcame this problem — and that of the heavy metals associated with commercial magnetic beads — by exploiting the paramagnetism of free radicals.

By submerging hydrogels in a stable radical solution, they were able to paramagnetize them, and then use permanent magnets to create fields capable of controlling them, without the need for external power. The result was an assembly of complex constructs with diverse and tunable material properties.

Credit: NPG

The team performed mechanical compression tests to determine the Young's moduli of the gels, and showed that their technique could be used to levitate micro-scale objects. They verified the viability of encapsulating cells in the gels, with an eye on possible biomedical applications. Their gels can be stained, studded with cells, or otherwise engineered, all before being exposed to radicals.

One of the most appealing aspects of the technique is that the magnetization used to induce self-assembly can be switched off with the application of an antioxidant like vitamin E. This inbuilt control is particularly good news for tissue-engineering applications that may be incompatible with the invasive and potentially ineffective methods for nanoparticle expulsion associated with other approaches.

The applications are many and varied. Tissue engineering and soft robotics are obvious examples, but the technique might also be useful for engineering radicals with improved magnetic properties. Most importantly for some, the authors were able to play Tetris with their system — showcasing the exquisite control afforded by their approach.