Analytical techniques for soft-solid characterization

G.J.C. Braithwaite, J. White
Cambridge Polymer Group, Inc.,
United States

Keywords: hydrogels, soft solids, non-newtonian, yield stress, biomimetic, swelling pressure, yield stress, organogels, elastomers


Analytical techniques for characterizing mechanical properties of conventional polymers are well known, including conventional tests such as tensile, compression, shear, and fatigue. As researchers develop materials with more targeted applications, analytical techniques need to become more specialized. Polymers and elastomers and more complex “soft-solids” like gels and creams, even relatively “rigid” materials, all exhibit temperature and time/strain dependences that are complex and fascinating, all wrapped in a material that is generally lighter and easier to work than equivalent metals. Often the key to the performance advantage of these materials arises out of their behavior as viscoelastic solids. In other words, the deformation response of these materials is highly rate- or time-dependent. This behavior arises out of mobility of the components within the solid, which in turn generates a rich and complex response to stimuli and opens up new applications that could not have been addressed with conventional rigid plastics or metals. This richness however comes with a difficulty from an engineering perspective, where conventional material testing techniques can be difficult to interpret or impossible to run on these interesting materials. Frequently the simple linear models used for conventional materials can not be applied to these soft solids. Even more important is that the end-use of the int3ended device must be accounted for in the testing. For example, if an elastomer is to be used as a vibration damper, a static creep test cannot address the critical performance needs of the material In this presentation we discuss a range of analytical tools, often based on conventional techniques, that can be used to probe fundamental properties and dynamic behaviors. Hydrogels are the archetype of soft solids. Characterization techniques must account for fluid flow into and out of the gel, as well as the delicate temperature response of these systems and, in many cases, the response of the material to external stimuli . Here we will discuss case studies from the development of three hydrogel medical devices, two injectable and one implantable, to highlight characterization techniques critical to their development. Rheological characterization of hydrogels, which is a direct measurement of their viscoelastic behavior, is an important starting point. This technique can probe yield strength and frequency-dependent responses. The latter just as important for rubbers (vibration damping, for example) as it is for gels. This technique is arguably common to all hydrogels. More specifically in the context of the injectable materials, we will discuss the swelling pressure of these materials, a parameter that is hard to measure conventionally but critical if the device is to be injected or if the material is being used as a seal. In the latter case, friction and wear may also be important, and we will discuss that in the context of implantable cartilage replacements. Using the same example, we will also discuss the challenges in measuring tensile and fracture toughness in these complex materials.