This goal of this research program is to develop a detailed understanding of the creasing instability induced by swelling of surface-attached polymer gels. Though this phenomenon has been observed in a variety of contexts since at least the mid-1800's, it remains poorly characterized and not widely appreciated. However, it has important implications for modern materials research in areas such as biomaterials and "smart" surfaces, and offers unique opportunities for the design of surfaces with switchable properties. The PI's work will focus on model hydrogel systems that allow quantitative determination of the following: (i) the conditions under which creasing occurs and how the onset of the instability depends on the material properties of the soft surface; (ii) the structures naturally adopted by creases, and how these structures can be controlled by proper material design; and (iii) how surface properties are altered by crease formation. Using the fundamental understanding gained through these experiments, responsive surfaces that dynamically alter their topography and chemistry in response to environmental cues will be designed. This research will be integrated into a variety of educational experiences in the laboratory and the classroom for students at the graduate, undergraduate, and K-12 levels.
NON-TECHNICAL SUMMARY
Coating a material with a thin layer of polymers provides a powerful way to control its surface properties. For example, surface-attached hydrogels (soft polymeric solids consisting primarily of water) are used to tune the interactions of materials with biological cells, and to create "smart" materials that sense and respond to changes in their environment. Under certain conditions, these soft polymer layers can undergo an instability in which their free surfaces spontaneously buckle and fold to form topographical features. This project aims to develop a thorough understanding of this instability, and its importance for surface properties, with significant impacts expected on the way that materials at the biological interface are designed, and on the creation of surfaces that can alter their properties on demand. In addition to providing technical training for undergraduate and graduate students in interdisciplinary areas of materials research, this program will offer the next generation of science educators at UMass an opportunity for a solid introduction to effective teaching strategies. Through a partnership with the Boston Museum of Science, the scientific results from this program will be incorporated into modules that reach a broader audience beyond the university.
Intellectual Merit: Over the course of this project, we made great strides in both understanding and applying the elastic creasing instability of soft polymer materials under compression. When the project began, creasing was a largely mysterious phenomenon that had appeared in disconnected places throughout the scientific literature, with implications in contexts including the formulation of soft biomaterials, the failure of tires and dielectric elastomers, and even the growth of biological tissues. Our experimental studies validated the predictions of elastic models studied by our collaborators and others, clearly establishing that creases cannot be understood based on a classical treatment of linear stability, but instead form via a nonlinear process directly as sharply folded singular features. Furthermore, we showed that the singular nature of a crease, combined with the stabilizing influence of surface tension, gives rise to an energy barrier against crease formation. This leads to a nucleation and growth behavior of creases, in a process closely analogous to the formation of cracks and crystal nuclei. Broder Impacts: In concert with these fundamental studies, we developed surfaces that take advantage of creasing to yield interesting and switchable surface properties. In particular, we showed that creasing has important implications for cell culture, and developed surfaces with switchable chemical patterns, enabling on-demand changes in adhesion, bioactivity, and enzymatic properties. We developed a variety of material systems that allow for switching of surface properties with stimuli including changes in temperature, light intensity, and applied electric potentials. Our work resulted in 10 peer-reviewed publications, including in high impact journals, along with more than 20 invited and contributed lectures by the PI, students, and post-docs at national and international conferences. The project provided support for the research of a diverse group of researchers, and has been instrumental in the progress of three PhD students, two post-doctoral fellows and eight undergraduate students. Alongside these research efforts, the project supported the development of a new graduate-level course that provides training in teaching skills for science and engineering PhD students, and the establishment of an outreach partnership with the Boston Museum of Science that offers museum visitors opportunities to learn about basic principles of polymer science and the importance of these materials in their lives.
