Hydrogel Tensile Testing
Hydrogel testing is most commonly done in both tension and compression. Compression testing on hydrogels poses less of a challenge, as many hydrogels are compliant and compress easily under load. The natural compliance of hydrogels becomes more of a challenge in tensile testing, as these materials can be difficult to grip and exhibit high elongation. Gripping hydrogels with too much pressure typically displaces the material out of the grip faces, which makes gripping ineffective. In addition, a traditional contacting extensometer is not a viable option considering the soft properties of hydrogels. Given the viscoelastic properties of these materials, test speed will greatly affect results, such as force at break and elongation at break.
When testing hydrogels in compression and tension, most common forces at failure are below 100 N. Given the low forces obtained in hydrogel mechanical testing, it is important that an accurate load cell is used. This can be especially tricky in a compression test as many users restrain from using low capacity load cells out of fear that they will overload the load cell if the compression platens begin to touch during a test. To prevent this from happening, it is imperative that safety limits are set and that the end of test criteria is set to the load cell capacity in the software. For tensile testing, Instron® offers a range of low force grips, including spring loaded, screw side action action, and pneumatic side action grips. Given the compliant nature of hydrogels, often sandpaper needs to be used to increase friction at the gripping contact points. When accurate measurement of strain is required, we recommend using our Advanced Video Extensometer.