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.