Chemomechanics of Soft Gels (Part I of the Invited Talk)

          Gels that consist of crosslinked polymeric network and solvent are ubiquitous in nature from cells, tissues to organs. They are also important engineering materials. For instance, various gels have been widely used as cell culture scaffold, drug carrier, microfluidic device, sensors, actuators, fuel cell membrane, and many others. With the recent development of 3D printing, many new applications are also made possible, such as soft robots with complex geometries, active metamaterials, personalized medicine, and artificial organs. Accurate design and precise control in the applications require a comprehensive model. However, the multi-phase and multi-physics nature of gels pose many challenges in modeling. Defying the classical definitions of solid and fluid, gels are both solid-like and fluid-like. The liquid component also provides an ideal media to host chemical reactions. The coupled liquid flow, chemical reaction, and network deformation makes the response of the materials sufficiently complex that ample room exists for new understandings connecting mechanics, chemistry and materials. In this work, we present a general nonequilibrium thermodynamic framework that couples large deformation, diffusion, chemical reaction and electrochemistry for gels. We then give a specific physics-based free energy function for photo-chemically responsive gels. This multi-physical model is not only useful in achieving accurate design and precise control in engineering applications, but also critical in unraveling new physical phenomenon in these complex materials. For instance, we will show that the volume phase transition temperature of the Poly(n-isopropylacrylamide) hydrogel decorated with photo-ionizable groups can be shifted using light, a phenomenon that was observed before, but only recently explained quantitatively in using this theory. This model can also facilitate new design of multifunctional materials with unique chemomechanical properties. Particularly, we will demonstrate a new photo-responsive hydrogel that has enhanced photo-efficiency and decoupled process of light activation and shape morphing for precise geometric control. The same system also exhibits opto-ionic response and is used to demonstrate a rewritable soft touch pad as the computer-humane interface.

          The full quantitative potential of the model in facilitating engineering designs and unraveling new physics also relies on accurate characterization of the thermodynamic and kinetic properties of the gels in the model. Despite the importance, experiments that can promote these quantitative studies of the diffusion-coupled-deformation behaviors of gels are still lacking due to many practical and theoretically challenges. Gels are extremely soft and have to be kept hydrated. Some of them are also slippery, brittle and saggy. Conventional mechanical testing methods are very difficult to be applied on gels. Additionally, the properties of gels are also size dependent. Comparing the existing mechanical testing techniques, indentation is most suitable for gels. It requires minimum sample preparation, can be easily performed in under water conditions, and can be easily carried out in different length scales just by changing the size of the indenter. The difficulty for using indentation is to accurately interpret the material properties from the measurement data. It is even challenging when the material exhibits coupled deformation and diffusion behaviors. In this work, we will demonstrate an oscillation indentation method to characterize the mechanical and transport properties of gels. Within the theory of poroelasticity, we show that a unified solution could be obtained for cylindrical punch, spherical indenter and conical indenter. The solutions are summarized in remarkably simple forms allowing the material parameters including both mechanical and transport properties to be extracted with ease. The method is demonstrated on various gels using AFM.