Preparation and Mechanical Characterisation of Self-Compressed Collagen Gels

Abstract

Collagen gels hold great promise in the field of tissue engineering as collagen is highly biocompatible, biodegradable, abundant in nature and it provides an optimum environment for tissue regeneration and restoration of normal tissue function. However, collagen hydrogels have poor mechanical strength due to low collagen proportion and thus are not capable of substituting native tissue without special treatment. The latter usually involves methods that impart some degree of cytotoxicity and impede optimal regeneration. In this study, hyper-hydrated collagen gels were concentrated without using any method that reduces cellular activity. Gels were left to self-compress in a laterally confined manner for a considerable period of time to expel excessive interstitial fluid and to transform to relatively concentrated collagen sheets. These collagen constructs may be seeded with cells and constitute an excellent starting material for building a tissue. The main goal of the study is to create and mechanically characterise self-compressed collagen gels, identifying the mechanical effect of expelling fluid by taking into account their two-phase nature. Plastically compressed collagen gels of three different concentrations were tested under unconfined ramp hold compression assuming biphasic theory. A finite element (FE) model was developed to simulate the experiment and analyse results by numerically fitting a solution to experimental data. The FE model was fitted to the experimental results using a numerical iteration algorithm to predict the values of material parameters. The collagen matrix was modelled as a neo-Hookean material, isotropic and homogeneous. Permeability of collagen gels was assumed to follow the strain dependency of Lai and Mow (1980). Collagen samples were also tested under dynamic loading to explore the frequency dependence of phase lag (?), storage and loss modulus. Results indicate that after confined self-compression for 18 hours, collagen density of gels increased almost 10 times, Young’s modulus ranged from 0.76-1.1kPa and zero-strain hydraulic permeability decreased from 51 to 21 mm4/Ns with increasing collagen content. The FE model coupled with the optimisation algorithm can detect differences in material parameters among gels of different collagen concentration and can reveal the poromechanics during loading. Further, dynamic mechanical analysis (DMA) revealed a profound increase of phase lag (?) and dynamic modulus with increasing frequency. The present work is the first work that studies the mechanical properties of concentrated collagen gels using biphasic theory. It constitutes a strong base from which more complex constitutive behaviour can be applied to the FE model. Although, the collagen concentration method via confined self-compression that was adopted did not result in collagen constructs strong enough to substitute native tissue, further compression of those materials in a laterally confined controllable manner could increase collagen density and mechanical properties, even in the range of body tissues.