Extremely tough hydrogel composites

Hierarchical structures within natural materials are widely known to provide advantageous combinations of stiffness, toughness, and friction. The combination of polyampholyte hydrogels and woven fabrics offers an attractive route to taking advantage of multi-scale interactions and advantageous swelling histories to provide a new class of composite materials with properties that match those of wet biological tissues.

HFSP Program Grant holder Alfred Crosby and colleagues
authored on Mon, 07 December 2015

Developing synthetic materials that demonstrate the unique properties of wet biological tissues, such as ligaments, has posed a significant challenge. It has been difficult to develop materials and methods which result in functional, soft biological components, in part, because many of the required characteristics are contradictory. For example, they must be soft and slippery, yet capable of supporting large loads while containing water to increase biocompatibility. Other rigid materials, such as metals, stiff fabrics, and synthetic polymers have failed due to their poor biocompatibility; while hydrogels alone are not strong enough. This research proposes a new concept to develop materials that are extremely stiff, yet flexible and biocompatible, while containing approximately 50% of water by volume. The inherently flexible and high strength properties of fabric make it an excellent choice for developing this new material. The composite material produced by combining stiff, glass fiber fabrics with soft, tough hydrogels, aims to mimic the design of natural ligaments.

This research marks the first time that researchers have successfully produced an extremely tough hydrogel composite by strategically selecting a recently developed tough hydrogel that de-swells in water and combining it with glass fiber woven fabrics. Previous attempts to produce similar composite materials were hampered by the fact that common hydrogels swell in water and interact poorly with solid components, limiting the strength of the material produced. In the newly developed approach, a polyampholyte hydrogel was selected due to its tendency to de-swell, by as much as 30%, to an equilibrium swelling ratio, resulting in a compressive stress on the woven fabric.  Furthermore, the polyampholyte molecules interact strongly with the surface of the glass fibers, resulting in enhanced fiber-matrix adhesion.  This combination of compressive pre-stress and high intrinsic adhesion yields a composite material with an impressive range of properties, which surprisingly outperforms the properties of either individual component.

The composite material formed through this process has several unique mechanical properties that have not previously been demonstrated with water-containing materials. First, trouser tear tests were used to demonstrate a 500% increase in the maximum tear strength exhibited by the polyampholyte composite (tear resistance demonstrated in Figure 1), compared to fabric alone. The tear strength of a control composite, composed of a polyacrylamide hydrogel rather than polyampholyte, was less than that of the neat fabric. Furthermore, the fracture energy of the polyampholyte composite, measured as an effective Gc, is extremely high. For a 40 mm wide sample, a Gc of 250,000 J m-2 is measured. In comparison, Gc of the neat fabric is 75,000 J m-2, and Gc of the neat polyampholyte gel is about 3000 J m-2. In addition, we found that the polyampholyte composite supports more than 2.5 times the load per sample width when compared to the neat fabric or the polyacrylamide composite. The polyampholyte and polyacrylamide composites had fracture strains of 0.08 and 0.04, respectively. The fracture strain of neat glass fabric was 0.02, demonstrating that the composites are less brittle than glass fabric alone. The toughness of the material produced was also measured through tensile tests which demonstrated that the failure energy for the polyampholyte composite is about seven times greater than for the neat fabric or the control polyacrylamide composite. Although the composites are extremely tough and stiff in extension, measurements from a three-point bending test demonstrate that they are very flexible in bending, similar to many natural materials.

Figure 1: Glass fabric reinforced polyampholyte hydrogels are extremely resistant to tear.  On the macroscale the fibers dissipate energy through friction, and on the microscale the gel dissipates energy through associative bond rupture. 

The unique composite materials developed here exhibit mechanical properties that have not previously been demonstrated with water-containing materials, including being extremely tough and resistant to tearing, with both a high degree of stiffness and flexibility at the same time.  Through development of composites that reinforce the polyampholyte hydrogel with glass fiber fabrics, we found that the mechanical properties of the composite material greatly exceed the properties of either fabric or hydrogel alone. The mechanism employed to create these hydrogel fabric composites provides an important, new, general framework for developing high strength and extremely tough water-containing materials, which could be particularly useful for biological prosthetics as well as commercial applications such as tear-resistant gloves, bullet-proof vests, or puncture-resistant tires.

Beyond the strong technological impact of these materials, the development of this new composite material was important in pursuing the milestones of the HFSP project of Alfred Crosby (Polymer Science & Engineering, UMass Amherst, USA), Duncan Irschick (Biology, UMass Amherst, USA), and Walter Federle (Zoology, Cambridge, UK).  A primary goal of this HFSP project is to understand the impact of compliant, or soft, substrates on the evolution of sub-surface structures in organisms that use adhesion in locomotion.  In addition to field work to understand the range of substrates (leaves, trees, rocks, etc.) on which geckos and insects reside and how the substrates’ properties relate to an organism’s morphology, our team is developing synthetic substrates to test specific predictions related to a new scaling theory that has been shown to relate adhesive force capacity to the mechanics of the organism and substrate.  Using conventional materials to test these theories has been challenging due to the limited range of elastic, highly compliant, yet tough, materials that exist with tunable properties.  The new composites were developed to help provide a new platform on which future testing of the predictive scaling relationships can be accomplished, leading to new insight into how the mechanical environment influences structural developments in adhesion-reliant organisms.

Reference

Extremely tough composites from fabric reinforced polyampholyte hydrogels. King DR, Sun TL, Huang Y, Kurokawa T, Nonoyama T, Crosby AJ, Gong JP. (2015) Materials Horizons. 2: 584-591. DOI: 10.1039/C5MH00127G

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