One of the recent discoveries in biological adhesion is the counterintuitive observation that some biological adhesive bonds, called catch bonds, are strengthened by tensile mechanical force. Adhesion via catch bonds offers the ultimate in reversible adhesion; gripping strongly under load but detaching when force is removed. The objective of this research is to determine design principles for mechanically regulated reversible adhesives based on a catch bond used by bacteria that normally live harmlessly in human intestines. In particular, this project will address the importance of the concentration and strength of adhesive molecules, how they are mechanically connected to the surface, and how force is applied between surfaces. This objective will be met with a mixture of theoretical calculations, stochastic multiscale simulations and experiments that measure adhesive behavior of bacteria or purified bacterial components integrated into the surface of small objects.
Reversible adhesives are of technological interest. For example, like geckos walking up a wall, robots need to grip a surface under load but then let go again. Similarly, robots need to grip and release objects they manipulate. The medical community also wants adhesives for regulated drug delivery and for microfluidic diagnostic devices. The basic principles provided in this project can aid in the design of adhesives for drug-delivery nanoparticles, microfluidic devices, and medical robots. This project will also expose the pipeline of engineers to the stochastic principles and modeling techniques that become important at the nano-scale through exposure of underrepresented high school students and enrichment of the university curriculum.
Cells such as bacteria, blood cells, and the cells that make up human tissues all need to adhere to their surroundings, but also need to let go in order to move. Therefore, they require a strong and yet reversible mode of adhesion. Using a common bacteria as a model for cell adhesion, we learned how cells can adhere strongly but also reversibly. They bind through individual bonds that catch under force, but release when force is removed, like children’s finger traps. We learned that these act as nanoscale locking seatbelts that allow bacteria to detach or roll to allow motility at low flow, but lock on at high flow to avoid being washed out of the body. We also learned that bacteria resist detachment by high flow because they bind through multiple organelles called fimbriae that yield (stretch at constant force) elastically to distribute the load equally between many adhesive receptors, so that no one receptor is overpowered. The advantage was unique to elastic yielding and not observed for other types of soft elastic stretching, such as the behaviors displayed by soft springs or rubber. Together, catch bonds and yielding elastic tethers allow cells to bind robustly and reversibly in a wide variety of flow conditions. Many other types of cells also bind through catch bonds and elastic yielding structures, so these conclusions may be generalized to understand biological adhesion of blood cells, the cells that make up many human tissues, and many other bacteria. This work also contributed to the field of adhesive technology. Biological adhesion can provide many unique properties that have not been possible with man-made adhesives. Currently, no man-made adhesive exhibits catch-bond like behavior nor elastic yielding, although irreversible (plastic) yielding is common in soft adhesives. This work demonstrated the advantages of these behaviors if nonbiological polymers could be developed that display catch bonding or elastic yielding. As an alternative approach, we developed adhesive technology directly using the bacterial organelles we studied. The bacteria we studied bind reversibly to most human cells and tissues in wet conditions. An adhesive with this same ability would be ideal to allow medical microrobotics to better grip and release tissue for manipulation or motility. However, existing adhesive technologies either bind irreversibly, fail in wet conditions, or don’t bind to human tissue. We identified ways to make a ‘BioCatch’ adhesive by incorporating the organelles from bacteria into a thing film. This adhesive demonstrated sufficiently strong adhesion at the microscale, but not the milliscale, so in its current form, it would be useful only for very small robotics. This project also exposed the pipeline of engineers to the stochastic principles and modeling techniques that become important at the nano-scale through enrichment of the curriculum at the University of Washington, and involvement of many students in research experience in this area.
