Abstract
Hydrogels as scaffolds provide a suitable environment for the cells (biocompatibility, biodegradability). Their biomechanical properties are very important to provide not only direct support to the surrounding tissue but also provide a local microenvironment. There is an interest in composite hydrogels with hydroxylapatite or bioactive glass (BAG) for tuning of their bioactivity and biomechanical properties [1].
Hydrogels were prepared from a polysaccharide gellan gum (GG), dissolved in ultrapure water at 90°C under constant stirring to a final concentration of 2 wt.% GG. Sodium-free BAG (70 wt.% SiO2, 30 wt.% CaO) was synthesized using a sol-gel technique with particles of ∼100 nm, clustered to ∼10 µm large agglomerates [1]. The hydrogel composites were prepared by admixing up to 2–8 wt.% of BAG powder into a solution of GG during sonication, and pouring the hot BAG-GG suspension with following cooling to room temperature. Mechanical properties were evaluated using different protocols in creep (0.1 to 1.2 N), strain sweep (1 to 20 µm) and frequency scan (100 to 0.1 Hz) modes, with specimens immersed in water at 25°C.
Maximum load (or deformation) before breaking of scaffold materials is a very important material property but is rarely measured. Here creep experiments at different applied stresses were carried out first. These loads exert more proper stress on the scaffold material that results in deformation, which is not the same as during deformation in relaxation or stress-strain tests [2]. The second set of experiments was made at physiologically relevant conditions (1 Hz frequency and small amplitude-controlled deformation) [3].
Amount of 2% BAG was found to be sufficient to get nearly linear deformation in the whole measured strains region, but at higher concentration stress deviated from linearity at strains exceeding ∼0.5% at 1 Hz. Storage modulus (E') did not significantly change and the loss tangent was found nearly constant (∼0.1) for the whole frequency range, indicating a strong network structure of BAG-doped hydrogel. Additions of 2% BAG give a ten-fold increase in both storage and loss moduli, whereas further increase of BAG content does not show further stiffening. The application of tailored protocols [3] allowed analysis of dynamic, creep and relaxation tests in the same device with same specimens, which might be not possible for other techniques.
Creep data would provide valuable information in addition to dynamic modes to predict long-term behaviour of the composite hydrogels. Properly tailored protocols could mimic, for example, articular cartilage or other tissue working conditions and allow evaluation of the side effects like swelling at early stage, which measurements are usually rather cumbersome.
