This talk present a new approach to study both the 2d-forward kinetic of bio-complexes formation and their mechanical properties (bending, streching and compression). Self-assembling of magnetic colloids under magnetic field allows to impose colloidal distances and forces.Therefore the distance between grafted biomolecules can be monitored during contact duration and the corresponding average time scale of the complex formation can be directly measured by counting the number of doublets that persist after the field is switched off. We present measurements of the 2d-forward rate kinetic of the complex IgG/Growth Hormon in Human Plasma. In the second part of the talk we show how long magnetic filaments made of linked magnetic particles may be used to measured the bending rigidity of linkers at a single link level. Indeed, Longflexible filaments made of magnetic colloids and linkers are shown to adopt under magnetic field a hairpin configuration. Measuring the hairpin curvature as a function of the field intensity and the linker length from diffracted light allows to deduce the linker bending rigidity.We have applied this approach to the bio complex IgG von-Willebrand Factor-IgG. We compare the bending rigidity of that complex to the one of actin or microtubule filaments.