Transition from viscous to elastic‐based dependency of mechanical properties of self‐assembled type I collagen fibers

Abstract

Fibrous collagen networks are the major elements that provide mechanical integrity to tissues; they are composed of fiber forming collagens in combination with proteoglycans and elastic fibers. Using uniaxial incremental tensile stress-strain tests we have studied the viscoelastic mechanical properties of self-assembled collagen fibers formed at pHs between 5.5 and 8.5 and temperatures of 25 and 37°C. Fibers formed at pH 7.5 and 37°C and crosslinked by aging at 22°C and 1 atmosphere pressure were also tested. Analysis of the mechanical tests showed that the ultimate tensile strength (UTS), and slopes of the total, elastic and viscous stress-strain curves were related directly to the volume fraction of polymer. Further analysis suggested that the UTS, and slopes of the total, elastic, and viscous stress-strain curves showed the highest correlation coefficient with the calculated effective fibril length and axial ratio. The mechanical data suggested that at low levels of crosslinking the mechanical properties were dominated by the viscous sliding of collagen molecules and fibrils by each other, which appears to be dependent on the collagen fibril length and axial ratio, while at higher levels of crosslinking the mechanical behavior is dominated by elastic stretching of the nonhelical ends, crosslinks, and collagen triple helix. The latter behavior appears to be dependent on the presence of crosslinks that stabilize fibrillar units. These results lead to the hypothesis that early in development viscous sliding of fibrils plays an important role in the mechanical response of animal tissues to forces experienced in utero, while later in development when locomotion is required, mechanical stability is primarily a result of elastic deformation of the different parts of the collagen molecule within crosslinked fibrils.

Failure of mineralized collagen fibrils: Modeling the role of collagen cross-linking

“…For collagen, sliding between individual collagen helixes is a relevant mode of deformation (Silver et al, 2003;Ja¨ger, 2005;Bu¨hler, 2006). Cross-linking inhibits sliding and favors elastic stretching modes of deformation (Silver et al, 2001). How this response of collagen translates into the response of bone has previously not been investigated.…”

Post-self-assembly experimentation on extruded collagen fibres for tissue engineering applications

“…In order to calculate the mechanical data, the stress at break was defined as the load at failure divided by the original cross-sectional area (engineering stress), whilst the strain at break was determined as the increase in fibre length required to cause failure divided by the original length. Collagen has been reported to exhibit viscoelastic properties up to 2% strain, where deformation of the triple helix is reportedly the predominant mechanism of deformation [23,36,41,42], whilst at strains beyond 2% increases in the D-period, increases in the gap region and relative slippage of laterally adjoining molecules along the fibre axis are observed [29,43,44]. Because of the nonlinear behaviour of collagenous structures, it would be incorrect to measure the Young's modulus, and thus we measured the secant modulus that was defined as stress at 2.2% strain divided by 0.022.…”

“…It is essential, however, that the physical properties of these materials are similar to the tissue that is being replaced [21]. Although some authors have investigated factors that influence the properties of these fibres either prior to [22][23][24][25] or during [26,27] the self-assembly, only the influence of different cross-linking methods and their biocompatibility have been investigated after self-assembly [25,[28][29][30][31][32][33][34], despite different washing baths, such as isopropanol [33,35], distilled water [36,37] or combinations of thereof [25,28,31], being used to treat the fibres with notable differences. We herein investigate for very first time the influence of different washing baths on the structural, thermal, physical and mechanical properties of the extruded collagen fibres.…”

A Mechanical Model of Porcine Vascular Tissues-Part II: Stress–Strain and Mechanical Properties of Juvenile Porcine Blood Vessels

“…Non-reducible cross-linking through enzyme activity is most likely the major contributor to the maturation process while glycation appears to result in degradation of ECM mechanical properties (Bailey et al, 1998). The normal increase in cross-linking activity occurs throughout the growth cycle, from almost purely viscous mechanics (in utero) to increasing elastic mechanical behavior, which provides tissues with the ability to withstand the normal physiological and environmental stresses applied to them (Silver et al, 2000). However, continuously increasing (locally, especially in the aorta) elastic behavior results in greater stresses in the peripheral vessels due to increased efficiency in transmitting pressure waves distally (Kelly et al, 1992).…”