Surface Viscometry of an Evaporating Droplet Containing a Protein (Collagen) as a Function of Time and Depth

There is a need for tissue engineers to recreate extracellular matrix that mimics the highly organized extracellular structures as seen in vivo. In a previous study in the extracellular matrix engineering research laboratory (EMERL), a micromechanical system was used to create such structures by drawing fibers from a droplet of neutralized collagen monomers at room temperature. For further investigation in the formation of highly aligned and continuous fibers, the laboratory is interested in developing a more effective experimental procedure. Therefore, to supply the proper concentration of collagen monomers for a collagen fiber printing device, a good estimate of the collagen concentration in the droplet surface is required. The goal of this study was to measure the concentration variation as a function of thickness in the dense layer on the droplet's top surface when a fiber can be created.;To measure the concentration, we need to know the viscosity of the droplet under the same experimental conditions. The viscosity of the droplet was estimated from the measured velocity of magnetic microspheres distributed in the collagen solution. To accelerate the microspheres the droplet was placed in the uniform region of a magnetic field produced by a permanent magnet. Magnetic microspheres travelled in the direction of the magnetic lines after quickly achieving with a constant velocity, which is related to the viscosity based on Stokes Law.;The average relative velocities in the direction of the magnetic field of these microspheres were measured using a custom MATLAB tracking algorithm at a depth of 20, 40, 60, and 1000 mum below the droplet surface. The concentration of the collagen was predicted based on a calibration curve relating the collagen concentration to the viscosity. The initial solution was made at 4.4 mg/ml of collagen monomers. After 150 seconds, the concentration inside the droplet (1000 mum below the surface) increased to 4.6 mg/ml, while the surface concentration spiked to 14 mg/ml. As expected, the concentration gradient is nonlinear from the droplet center to the surface. Between the surface to 20 mum below the surface, the collagen monomer concentration dropped from 14 mg/ml to 8 mg/ml. Therefore, the layer of dense collagen solution is limited to less than 20 mum below the surface.;The result of this study gave an important information in the critical surface concentration where collagen fibers can be formed, which can lead to the design of a more efficient and predictable methodology to produce highly organized collagen fibers.