Abstract
For a meaningful evaluation of biomaterials, in vitroenvironments that mimic the physiological properties of the in vivoenvironment are desirable with relevant control of key factors. For faster screening and reduction of time and costs, combination and control of different critical parameters are needed.
Commercial Hyalograft® and ChondroGide® scaffolds were compared to a new experimental recombinant human collagen-PLA (rhCo-PLA) [1] and pure PLA scaffolds under BEST protocol [2] in pseudostatic (creep), dynamic (frequency scans, strain sweeps), and combined conditions (simulated operative periods) relevant for orthopaedic applications. Temperatures 25–37°C, dry and fully immersed wet (water, 0.9% NaCl) conditions were analysed and aggregate, complex dynamic moduli and loss factor were obtained. Additionally a method was developed for estimation of the swelling pressure under variable compression. ChondroGide and rhCo-PLA were compared in vivoin earlier experiments [1].
All scaffold materials have a non-linear and non-uniform behaviour when immersed in a fluid, accompanied by rapid change in starting porosity (down for Hyalograft® and ChondroGide®, up for PLA), but nearly stable for rhCo-PLA. Too hydrophilic materials exhibited partial non-wetting (dry spots) under a slight compression eventually by closure of the specimen rim due to elastocapillary effect, where as hydrophobic (PLA) shown substantial expansion. The swelling pressure of PLA was measured of ∼1 kPa (water, 25°C). Observed creeping cannot be reliably fit with simple viscoelastic models, but can be approximated with biphasic theory with variable complex moduli and permittivity values. No significant differences were observed in creep for 1 h and 5 h runs, showing that a shorter time is sufficient to catch the main effects in these biomaterials. No substantial differences were observed between water and NaCl solution at 37°C, except for ChondroGide® which swells in NaCl more than in water. Besides of some differences in swelling, no significant differences observed between 25 and 37°C tests for creep. For dynamic conditions all materials undergo densification and “stiffening” (50% and more) upon cyclic strain deformation, with the effect being higher at 37°C than at 25°C. rhCo-PLA scaffolds exhibit relatively stable modulus in water and loss factor with physiologically-compatible behaviour (∼0.1 with a minimum values range around 1 Hz) at frequency scans (0.01–20 Hz). On the contrary, ChondroGide® has the highest loss factor (up to 0.6–0.7).
Water at 25°C seems to be sufficient to rapidly test these kinds of materials for biomechanical screening, unless additions or specific effects are of interest. The applied deformation level is more important to predict materials properties in dynamic conditions than experiment time. This means that better in vitrodata can be obtained in shorter runs. The animal studies have also exhibit rhCo-PLA producing better quality (ICRS median score 12.5 vs. 8.5 for ChondroGide®).
