This article reports on measurements of the rheological properties of slightly compressible collagen liquid (from 6.6 to 8.0% mass fraction of native bovine collagen in water) using a capillary rheometer. The extrusion rheometer is equipped with an annular slit between a circular tube and a central pin. The sample of collagen to be tested is pushed by a piston from a container to the annular measuring section. A manually adjusted hydraulic drive enables the piston velocity and the corresponding shear rate to be varied continuously within the range from 50 to 3000 s 21. Strain gages installed in the outer cylinder monitor the axial pressure profile. It is therefore possible to identify not only the basic nonelastic shear flow characteristics (Herschel Bulkley and Power law model parameters), but also the first normal difference evaluated from the exit pressure. The results of experiments carried out at room temperature show rather broad variability of the parameters. PRACTICAL APPLICATIONSThe rheological properties of native collagenous materials are of primary importance in the design of extrusion processes, for example, extrusion of sausage casings (a description of the flow behavior not only inside the screw extruders, but also while the extruded tubes are being blown) and similar technologies (injection molding, film blowing, rolling, and so forth). Knowledge of the rheological properties of native collagenous vascular grafts is important for an indirect assessment of irradiation effects and the effects of additives on a collagenous matrix (crosslinking). Native collagenous materials are rather "awkward" from the point of view of rheometry. For example, rotational rheometers are limited to a linear oscillation regime (due to discharge of the samples and shear banding), and the application of capillary rheometers is complicated by the inhomogeneity of native collagen (presence of bubbles). The proposed rheometer takes into account the compressibility of the tested samples, and enables an evaluation not only of the shear properties but also of the elastic properties within the range of deformation rates corresponding to the production lines.
Although collagen is widely used (for example, in the food industry, in the pharmaceutical industry and in biomedicine), the rheological properties of the material are not well known for high concentrations (8% collagen, 90% water). Rheological properties were measured using a capillary-slit rheometer (an extrusion process), where the tested sample of collagen matter was pushed by a hydraulically driven piston through a narrow rectangular slit at very high shear rates of 50–6 000 s<sup>–1</sup>. The Herschel-Bulkley (HB) constitutive equation and a new correlation taking into account the finite gap width was used to evaluate the rheological properties (n = 0.2, K = 879 Pa s<sup>n</sup>, τ<sub>0</sub> = 2 380 Pa). Use was made of a new yield stress measurement method evaluating τ<sub>0</sub> 'post mortem' after extrusion stops. The effects of wall slip and of air bubbles, which caused apparent compressibility of the 'silly putty' collagen material, were also studied. Corrections of the wall slip effect were implemented using sliding layer thickness δ.
This article describes 1D extension tests on bovine collagen samples (8% collagen in water). At such a high collagen concentration, the mechanical properties of semi-solid samples can be approximated by hyperelastic models (two-parametric HGO and Misof models were used), or simply by Hooke’s law and the modulus of elasticity E. The experiments confirm a significant increase in the E-modulus of the samples irradiated with high-energy electrons. The modulus E~9 kPa of non-irradiated samples increases monotonically up to E~250 kPa for samples absorbing an e-beam dose of ~3300 Gy. This amplification is attributed to the formation of cross-links by irradiation. However, E-modulus can be increased not only by irradiation but also by exposure to a high strain rate. For example, soft isotropic collagen extruded through a 200 mm long capillary increases the modulus of elasticity from 9 kPa to 30 kPa, and the increase is almost isotropic. This stiffening occurs when the corrugated collagen fibers are straightened and are aligned in the flow direction. It seems that the permanent structural changes caused by extrusion mitigate the effects of the ex post applied irradiation. Irradiation of extruded samples by 3300 Gy increases the modulus of E-elasticity only three times (from 30 kPa to approximately 90 kPa). Extruded and ex post irradiated samples show slight anisotropy (the stiffness in the longitudinal direction is on an average greater than the transverse stiffness).
This paper focuses on measurements of the electrical properties, the specific heat capacity and the thermal conductivity of a collagen solution (7.19 % mass fraction of native bovine collagen in water). The results of our experiments show that specific electrical conductivity of collagen solution is strongly dependent on temperature. The transition region of collagen to gelatin has been observed from the measured temperature dependence of specific electrical conductivity, and has been confirmed by specific heat capacity measurements by a differential scanning calorimetry.
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