Finding the Connective Tissue of Soft Materials

The human body uses adhesion to hold itself together. For example, a tendon attaches muscle to bone, while connective tissue attaches muscle to skin. 

Hydrogel-based soft materials are based on these biomimetic mechanical behaviors, which makes them a revolutionary design of biomedical implants, human-machine interfaces, and bio-inspired soft robots. However, there are limitations to overcome before they are able to fully replace commonly used hard materials. 

Qihan Liu, assistant professor of mechanical engineering and materials science at the University of Pittsburgh Swanson School of Engineering, received a $546,127 Faculty Early Career Development Award from the National Science Foundation (NSF) to find a novel solution to adhesion in soft materials through thermodynamic interaction.  

“Professor Liu is an outstanding researcher and this CAREER award will bolster activities in at least two core research competencies in our Department of Mechanical Engineering and Materials Science, specifically advanced manufacturing and design and soft matter biomechanics,” said Brian Gleeson, department chair. 

Gellin’ It Together with Thermodynamics

Hard materials are typically assembled together with screws and interlocking structures, so a machine can be easily upgraded or repaired by changing parts. Currently, adhesion in soft materials is irreversible, so it is nearly impossible to upgrade or repair a soft machine like conventional machines. 

“Despite the extensive research on gel adhesion, there is a critical unmet need for stimuli-responsive adhesion that not only has a strong grip, but also detaches and reattaches and is reversible under designated stimuli like temperature, stress, and the presence of certain chemicals,” Liu explained. “We’re learning that thermodynamic interaction between hydrogels can make this possible.” 

Sticky When You Need It

Thermodynamic adhesion is distinct from existing studies of bonding-based adhesions. 

There are two known thermodynamic adhesion mechanisms: osmocapillary and electrostatic adhesion. Osmocapillary adhesion results from the suction generated when neighboring hydrogel absorbs the interfacial solvent, which is governed by the thermodynamics of osmosis. Electrostatic adhesion results from the attraction between oppositely charged polymer networks surrounded by free ions. 

“Both types of thermodynamic adhesion respond to a wide variety of stimuli and can be reversible, which is our main goal ,” Liu said. “We need to build a predictive model that can test thermodynamic adhesion under different conditions to determine if it's a feasible solution.” 

Using thermodynamic interaction for hydrogels can lead to a number of applications. For example, a smart watch can be applied to a wrist without a band and aid in invasive medical devices. 

In addition to the scientific contributions of adhesion in soft materials, Liu’s CAREER award will also help him develop a video style that is both easy for researchers to produce and interesting for laypeople to watch, thus promoting the direct dissemination of cutting-edge research to the general public through free video-sharing platforms. 

The five-year project, “Robust, Reversible, and Stimuli-responsive Thermodynamic Adhesion in Hydrogels,” is set to begin May 2024.