Mechanics of semiflexible chains formed by poly(ethylene glycol)-linked paramagnetic particles

Abstract

Magnetorheological particles, permanently linked into chains, provide a magnetically actuated means to manipulate microscopic fluid flow. Paramagnetic colloidal particles form reversible chains by acquiring dipole moments in the presence of an external magnetic field. By chemically connecting paramagnetic colloidal particles, flexible magnetoresponsive chains can be created. We link the paramagnetic microspheres using streptavidin-biotin binding. Streptavidin coated microspheres are placed in a flow cell and a magnetic field is applied, causing the particles to form chains. Then a solution of polymeric linkers of bis-biotin-poly(ethylene glycol) molecules is added in the presence of the field. These linked chains remain responsive to a magnetic field; however, in the absence of an external magnetic field these chains bend and flex due to thermal motion. The chain flexibility is determined by the length of the spacer molecule between particles and is quantified by the flexural rigidity or bending stiffness. To understand the mechanical properties of the chains, we use a variety of optical trapping experiments to measure the flexural rigidity. Increasing the length of the poly(ethylene glycol) chain in the linker increases the flexibility of the chains.

Numerical study of a microscopic artificial swimmer

“…(22), introduced first by Wiggins and Goldstein [23,37] and termed sperm number by Lowe [26], is defined via…”

“…A static external magnetic field induces dipoles in the colloids so that they form a chain. In the gaps between the charged colloids chemical linkers such as doublestranded DNA are attached to the particles and an elastic filament resisting bending and stretching is formed [19,20,21] (for similar systems see [22]). As demonstrated by the impressive experiments of Dreyfus et al, an oscillating external magnetic field now induces a non-reciprocal beating motion of the superparamagnetic filament that is able to move the attached red-blood cell forward.…”

Directing Assembly of DNA-Coated Colloids with Magnetic Fields To Generate Rigid, Semiflexible, and Flexible Chains

“…Many types of molecules have been used as linkers, including bifunctional molecules 17,[25][26] , polymers adsorbed to the surface of the particle that then become entangled when the particles are brought together, 27-28 and short single-stranded DNA linker molecules. [29][30] Micron-sized colloids linked with small molecules, such as glutaraldehyde 17,25 and low molecular weight poly(ethylene glycol), 26 produce chains that are very stiff, where the persistence lengths mm) exceed the contour lengths. These linking chemistries can only produce chains in the rigid-rod regime and are severely limited with respect to the ability to create flexible polymer analogs.…”

Self-assembly of polymer-like structures of magnetic colloids: Langevin dynamics study of basic topologies

“…This provides a much higher resistance to mechanical stresses, a significantly increased magnetic response [29] and a total independence from the self-assembly conditions of the particles after the cross-linking, with the only potential downside of obtaining a much higher rigidity of the chains. The latter property, however, can be tuned by selecting the composition and length of the polymer crosslinkers [30,31]. Even this tuning has been achieved to date only for microparticles, cutting-edge experimental techniques like DNA designed self-assembly [32,33] or in situ polymer mineralisation [34,35] open up the possibility to also create rather flexible linear polymer-like chains of magnetic nanoparticles.…”