Hydrogels, mechanical measurements

Mechanical measurements on hydrogels can easily be performed by TMA in compression in a liquid environment at various temperatures. In one study, the swelling behavior of hydrogels made from polysaccharide, calcium pectin, and cellulose ethers was studied by TMA (lijima et al. 2005 Ford and Mitchell 1995). In these studies, samples with varying crosslink densities were immersed in water, and creep behavior was characterized as a function of immersion time and temperature using a compression-type probe. As expected the creep increased with increasing swelling tenaperature and swell ratio. 

Direct mechanical methods can be used to determine the swelling pressure of hydrogels, e.g., by means of devices in the form of a cylindrical chamber equipped with a piston in which the gel contacts the solution through a porous membrane. This technique allows measuring very low pressure (of the order of 0.1-10 kPa) and makes it possible to analyze the SAH with swelling up to 700 ml g-1 [102, 103]. Among others, the method of osmotic deswelling is to be mentioned [104]. 

Characterization of Hydrogel Films. Mechanical testing was conducted in buffered saline on an Instron instrument, according to the modified ASTM D-1708 (tensile) and D-1938 (tear) and were reported in g/mm2 for modulus and g/mm for tear strength. The water contents and the amount of extractables were measured gravimetr ica1ly. 

In this study, uniaxial confined swelling and compression experiments were performed on a hydrogel that mimics the behaviour of biological tissues. The deformation of the sample and the electrical potential difference over the sample, caused by varying mechanical and chemical loads, were measured successfully. 

The mechanical behavior of the hydrogels can be described by the theories of rubber elasticity and viscoelasticity, which are based on time-independent and time-dependent recovery of the chain orientation and structure, respectively. Mechanical properties due to rubber elastic behavior of hydrogels can be determined by tensile measurements, while the viscoelastic behavior can be determined through dynamic mechanical analysis. 

Practically, tensile samples were allowed to swell in Millipore water for 24h in order to reach the equilibrium prior mechanical testing. In such conditions, only strain due to mechanical deformation, and not to swelling process, was measured. Figure 11 presents typical true stress-strain curves. It comes out that the introduction of PCL segments enliances the overall mechanical properties in comparison to the PDMAEMA hydrogel. 

Both physical absorption and covalent bonding mechanisms were investigated for immobilizing the lipase onto the PAA/PVA hydrogel fibrous membrane. For the covalent bonding mechanism, EDC was used as the coupling reagent. All measurements were performed in triple. 

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