Advanced Life Science
Developing Extremely Tough Soft Composites
As the strength of a material increases, in general, it will become increasingly brittle and prone to catastrophic failure. This trade-off has major implications when designing materials, as often weaker materials are used to prevent uncontrolled fracture. Notably, many biological materials exhibit enhanced toughness across a wide range of moduli, from soft, water-containing materials such as ligament to very stiff materials such as nacre, found in seashells. Both of these examples are naturally-occurring composite materials, and while the individual components which make up these materials are not exceptional, the unique design of these composites results in a synergistic increase in mechanical properties. Our goal is to utilize the advances of modern materials science, along with preferential designs from nature to create new composite materials with extreme mechanical properties and functionalities.
Many biological applications require specific yet often contradictory properties, such as stiffness, flexibility, and shock absorption, all while containing water. As a base towards achieving these properties, we have utilized polyampholyte hydrogels as a tough, dissipative matrix material. To enhance the mechanical properties, we reinforce the hydrogels with materials such as woven fabrics, 3d printed patterns, and liquid crystalline polymers. Each of these materials provides us with a different method for tuning the mechanical properties of the composite, by either varying chemistry or geometry.
The process of combining two very different materials together does not inherently result in robust composite materials. We have discovered that to develop mechanically robust composite materials two criteria must be satisfied: a strong interface must exist between the reinforcing material and the matrix (through chemical/physical bonding, mechanical interlocking, or pre-stress), and the matrix must possess an ability to dissipative energy to prevent catastrophic failure of the reinforcing material during loading. We have demonstrated this understanding through the development of extremely tough polyampholyte hydrogel - woven glass fabric composites. The polyampholyte is able to bond strongly with glass, and dissipates energy through recoverable fracture of ionic bonds. These materials exhibit a strongly synergistic increase in toughness (~10x) over either individual component. They also possess increased tensile fracture stress (~3x) compared to neat fabrics, yet maintain high anisotropic modulus (1000x stiffer in extension than bending), which is inherent to fabrics (revealed through the ability of “drape”). Importantly, we show that composite toughness scales with matrix toughness. This opens up the possibility for developing increasingly tough, soft materials for future biomedical applications.