Originally developed for biophysical research, optical methods such as video microscopy, Diffusing Wave Spectroscopy (DWS) nowadays are more and more used to determine viscoelastic properties of various kinds of complex fluids. The basic idea of optical microrheology is to study the equilibrium thermal response of small (colloidal) particles embedded in a material and thereby obtain quantitative information about the macroscopic storage and loss moduli, G´(ω) and G´´(ω).
DWS is a fast and powerful technique working without external driving force and able to measure viscoelastic properties of transparent fluid over an extended range of frequencies (0.001-100 kHz)
DWS possibilities and limits are validated in our group in comparing systematically results with a mechanical high frequency rheology technique. DWS has been successfully used for the investigation of viscoelastic characteristics of semiflexible polymers and selfassembled structures. We succeeded for the first time to determine the bending stiffness of semiflexible micellar solutions from rheological measurements. DWS has also been successfully applied to study local structural variations or high frequency relaxation processes e.g. in polymeric thickeners, protein solutions, starch composites, clay suspensions, highly concentrated polymer dispersions and intermediate filament networks.
The principle of video microscopy consists of monitoring the thermally driven motion of inert microspheres that are evenly distributed within the solutions and to statistically analyze the distribution of mean square displacements, from which information about the extent of heterogeneity and rheological properties can be extracted. This technique has been used for the characterization of inhomogeneous polymer solutions, human tracheal mucus, DNA hydrogels, thermosensitive resin systems and supramolecular hydrogels.
Recently, we focused on biopolymers like hyaluronic acid (HA) solutions and developed a novel strategy to fabricate porous HA-copolymer hydrogels including collagen or chitosan aiming at their application in tissue engineering. Using Multiple Particle Tracking (MPT), viscoelastic properties of the gels can be determined locally at the site of cell attachment and latter can be correlated to the cell growth. Such substrates are developed in cooperation with Prof. Bastmeyer (KIT) for the growth and proliferation of skin cells.